Implants using ultrasonic waves for stimulating tissue

ABSTRACT

Described herein are implantable devices configured to emit an electrical pulse. An exemplary implantable device includes an ultrasonic transducer configured to receive ultrasonic waves that power the implantable device and encode a trigger signal; a first electrode and a second electrode configured to be in electrical communication with a tissue and emit an electrical pulse to the tissue in response to the trigger signal; and an integrated circuit comprising an energy storage circuit. Also described are systems that include one or more implantable device and an interrogator configured to operate the one or more implantable devices. Further described is a closed loop system that includes a first device configured to detect a signal, an interrogator configured to emit a trigger signal in response to the detected signal, and an implantable device configured to emit an electrical pulse in response to receiving the trigger signal. Further described are computer systems useful for operating one or more implantable devices, as well as methods of electrically stimulating a tissue.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. Non-Provisional applicationSer. No. 16/313,865, filed on Dec. 27, 2018, which is a U.S. NationalPhase Application of International Application No. PCT/US2017/041264,filed Jul. 7, 2017, which claims priority to U.S. ProvisionalApplication No. 62/359,672, filed on Jul. 7, 2016, entitled “NEURAL DUSTAND ULTRASONIC BACKSCATTER IMPLANTS AND SYSTEMS, AND APPLICATIONS FORSUCH SYSTEMS,” the disclosure of each of which is incorporated herein byreference in its entirety and for all purposes.

STATEMENT AS TO FEDERALLY SPONSORED RESEARCH

This invention was made with government support under Grant Nos.HR0011-15-2-0006 awarded by the Defense Advanced Research ProjectsAgency (DARPA) and R21-E027570 awarded by the National Institute ofHealth (NIH). The government has certain rights in the invention.

TECHNICAL FIELD

The present invention relates to implantable devices operated usingultrasonic waves for emitting an electrical pulse or stimulating tissue.

BACKGROUND

The emerging field of bioelectronic medicine seeks methods fordeciphering and modulating electrophysiological activity in the body toattain therapeutic effects at target organs. Current approaches tointerfacing with peripheral nerves, the central nervous system and/ormuscles rely heavily on wires, creating problems for chronic use, whileemerging wireless approaches lack the size scalability necessary tointerrogate small-diameter nerves. Furthermore, conventionalelectrode-based technologies lack the capability to record from nerveswith high spatial resolution or to record independently from manydiscrete sites within a nerve bundle.

Recent technological advances and fundamental discoveries have renewedinterest in implantable systems for interfacing with the peripheralnervous system. Early clinical successes with peripheralneurostimulation devices, such as those used to treat sleep apnea orcontrol bladder function in paraplegics have led clinicians andresearchers to propose new disease targets ranging from diabetes torheumatoid arthritis. However, currently known neurostimulation devicesare generally fully external devices and unable to stimulate deeptissue, not fully implantable, or are unable to accurately stimulate anerve without risking off-target stimulation.

SUMMARY OF THE INVENTION

Provided herein are implantable devices configured to emit an electricalpulse to a tissue, systems comprising an implantable device and aninterrogator to operate the implantable device, and closed-loop systemscomprising a first device configured to detect a physiological systemand an implantable device configured to emit an electrical pulse to atissue in response to an interrogator receiving the physiologicalsignal. Further provided are computer systems configured to operate oneor more implantable devices. Also provided are methods of stimulating atissue.

In some embodiments, there is provided an implantable device, comprisingan ultrasonic transducer configured to receive ultrasonic waves thatpower the implantable device and encode a trigger signal; a firstelectrode and a second electrode configured to be in electricalcommunication with a tissue and emit an electrical pulse to the tissuein response to the trigger signal; and an integrated circuit comprisingan energy storage circuit. In some embodiments, the electrical pulse isa current pulse. In some embodiments, the electrical pulse is a voltagepulse.

In some embodiments, the first electrode and the second electrode arewithin the tissue or in contact with the tissue. In some embodiments,the tissue is muscle tissue, organ, or nervous tissue. In someembodiments, the tissue is part of the peripheral nervous system or thecentral nervous system. In some embodiments, the tissue is a skeletalmuscle, smooth muscle, or cardiac muscle.

In some embodiments, the integrated circuit comprises a digital circuit.In some embodiments, the integrated circuit comprises a mixed-signalintegrated circuit configured to operate the first electrode and thesecond electrode. In some embodiments, the integrated circuit comprisesa power circuit comprising the energy storage circuit.

In some embodiments, the implantable device comprises a body thatcomprises the ultrasonic transducer and the integrated circuit, whereinthe body is about 5 mm or less in length in the longest dimension. Insome embodiments, the body has a volume of about 5 mm³ or less. In someembodiments, the implantable device comprises a non-responsivereflector.

In some embodiments, the implantable device comprises three or moreelectrodes.

In some embodiments, the integrated circuit comprises ananalog-to-digital converter (ADC).

In some embodiments, the implantable device comprises a modulationcircuit configured to modulate a current flowing through the ultrasonictransducer. In some embodiments, the modulated current encodesinformation, and the ultrasonic transducer is configured to emitultrasonic waves encoding the information. In some embodiments, theinformation comprises a signal verifying that an electrical pulse wasemitted by the implantable device, a signal indicating an amount ofenergy stored in the energy storage circuit, or a detected impedance. Insome embodiments, the implantable device comprises a digital circuitconfigured to operate the modulation circuit. In some embodiments, thedigital circuit is configured to transmit a digitized signal to themodulation circuit. In some embodiments, the digitized signal comprisesa unique implantable device identifier.

In some embodiments, the ultrasonic transducer is configured to receiveultrasonic waves from an interrogator comprising one or more ultrasonictransducers. In some embodiments, the ultrasonic transducer is a bulkpiezoelectric transducer, a piezoelectric micro-machined ultrasonictransducer (PMUT), or a capacitive micro-machined ultrasonic transducer(CMUT).

In some embodiments, the implantable device is implanted in a subject.In some embodiments, the subject is a human.

In some embodiments, the implantable device is at least partiallyencapsulated by a biocompatible material. In some embodiments, at leasta portion of the first electrode and the second electrode are notencapsulated by the biocompatible material.

Also provided herein is a system comprising one or more implantabledevices and an interrogator comprising one or more ultrasonictransducers configured to transit ultrasonic waves to the one or moreimplantable devices, wherein the ultrasonic waves power the one or moreimplantable devices. In some embodiments, the ultrasonic waves encode atrigger signal. In some embodiments, the system comprises a plurality ofimplantable devices. In some embodiments, the interrogator is configuredto beam steer transmitted ultrasonic waves to alternatively focus thetransmitted ultrasonic waves on a first portion of the plurality ofimplantable devices or focus the transmitted ultrasonic waves on asecond portion of the plurality of implantable devices. In someembodiments, the interrogator is configured to simultaneously receiveultrasonic backscatter from at least two implantable devices. In someembodiments, the interrogator is configured to transit ultrasonic wavesto the plurality of implantable devices or receive ultrasonicbackscatter from the plurality of implantable devices using timedivision multiplexing, spatial multiplexing, or frequency multiplexing.In some embodiments, the interrogator is configured to be wearable by asubject.

Also provided herein is a closed-loop system, comprising (a) a firstdevice configured to detect a signal; (b) an interrogator comprising oneor more ultrasonic transducers configured to receive the ultrasonicbackscatter encoding the electrophysiological signal and emit ultrasonicwaves encoding a trigger signal; and (c) a second device configured toemit an electrical pulse in response to the trigger signal, wherein thesecond device is implantable, comprising an ultrasonic transducerconfigured to receive ultrasonic waves that power the second device andencode a trigger signal; a first electrode and a second electrodeconfigured to be in electrical communication with a tissue and emit anelectrical pulse to the tissue in response to the trigger signal; and anintegrated circuit comprising an energy storage circuit. In someembodiments, the signal is an electrophysiological pulse, a temperature,a molecule, an ion, pH, pressure, strain, or bioimpedance.

In some embodiments of the closed-loop system, the first device isimplantable. In some embodiments, the first device comprises a sensorconfigured to detect the signal; an integrated circuit comprising amodulation circuit configured to modulate a current based on thedetected signal, and a first ultrasonic transducer configured to emit anultrasonic backscatter encoding the detected signal from the tissuebased on the modulated current. In some embodiments, the sensorcomprises a first electrode and a second electrode configured to be inelectrical communication with a second tissue. In some embodiments, thefirst tissue and the second tissue are the same tissue. In someembodiments, the first tissue and the second tissue are differenttissues.

In some embodiments of the closed-loop system, the first electrode andthe second electrode of the second device are within the tissue orcontact the tissue. In some embodiments, the integrated circuit of thesecond device comprises a digital circuit. In some embodiments, theintegrated circuit of the second device comprises a mixed-signalintegrated circuit configured to operate the first electrode and thesecond electrode. In some embodiments, the integrated circuit comprisesa power circuit comprising the energy storage circuit.

In some embodiments of the closed-loop system, the tissue is muscletissue, an organ, or nervous tissue. In some embodiments, the firstdevice and the second device are implanted in a subject. In someembodiments, the subject is a human.

Further provided herein is a computer system, comprising an interrogatorcomprising one or more ultrasonic transducers; one or more processors;non-transitory computer-readable storage medium storing one or moreprograms configured to be executed by the one or more processors, theone or more programs comprising instructions for operating theinterrogator to emit ultrasonic waves encoding a trigger signal thatsignals an implantable device to emit an electrical pulse to a tissue.In some embodiments, the interrogator is operated to emit ultrasonicwaves encoding the trigger signal in response to a detectedphysiological signal. In some embodiments, the physiological signalcomprises an electrophysiological pulse, a temperature, a molecule, anion, pH, pressure, strain, or bioimpedance. In some embodiments, the oneor more programs comprise instructions for detecting the physiologicalsignal based on ultrasonic backscatter encoding the physiological signalemitted from a second implantable device. In some embodiments, the oneor more programs comprise instructions for determining a location ormovement of the first implantable device or the second implantabledevice relative to the one or more ultrasonic transducers of theinterrogator.

Also provided herein is a method of electrically stimulating a tissue,comprising receiving ultrasonic waves at one or more implantabledevices; converting energy from the ultrasonic waves into an electricalcurrent that charges an energy storage circuit; receiving a triggersignal encoded in the ultrasonic waves at the one or more implantabledevices; and emitting an electrical pulse that stimulates the tissue inresponse to the trigger signal. In some embodiments, the trigger signalis transmitted in response to a detected physiological signal.

Further provided is a method of electrically stimulating a tissue,comprising emitting ultrasonic waves encoding a trigger signal from aninterrogator comprising one or more ultrasonic transducers to one ormore implantable devices configured to emit an electrical pulse to thetissue in response to receiving the trigger signal. In some embodiments,the trigger signal is transmitted in response to a detectedphysiological signal.

Also provided herein is a method of stimulating a tissue, comprisingreceiving ultrasonic waves at one or more implantable devices configuredto detect a physiological signal; converting energy from the ultrasonicwaves into an electrical current that flows through a modulationcircuit; detecting the physiological signal; modulating the electricalcurrent based on the detected physiological signal; transducing themodulated electrical current into an ultrasonic backscatter that encodesinformation related to the detected physiological signal; and emittingthe ultrasonic backscatter to an interrogator comprising one or moretransducer configured to receive the ultrasonic backscatter; emittingultrasonic waves from the interrogator to one or more implantabledevices configured to emit an electrical pulse to the tissue; convertingenergy from the ultrasonic waves emitted from the interrogator to theone or more implantable devices configured to emit the electrical pulseinto an electrical current that charges an energy storage circuit;emitting ultrasonic waves encoding a trigger signal from theinterrogator; receiving the trigger signal at the one or moreimplantable devices configured to emit the electrical pulse; andemitting an electrical pulse that stimulates the tissue in response tothe trigger signal.

In some embodiments of the method of stimulating a tissue, thephysiological signal comprises an electrophysiological pulse, atemperature, a molecule, an ion, pH, pressure, strain, or bioimpedance.

In some embodiments of the method of stimulating a tissue, the tissue isa muscle tissue, an organ, or a nervous tissue.

In some embodiments of the method of stimulating a tissue, the methodcomprises implanting the one or more implantable devices in a subject.In some embodiments, the subject is a human.

In some embodiments of the method of stimulating a tissue, the methodcomprises determining a location or movement of the one or moreimplantable devices.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic of a neural dust system, including an externaltransceiver, a sub-dural interrogator, and a neural dust mote, asdescribed in Seo et al., Neural dust: an ultrasonic, low power solutionfor chronic brain-machine interfaces, arXiv: 1307.2196v1 (Jul. 8, 2013).

FIG. 2A is a block diagram of an exemplary interrogator for a systemdescribed herein. The illustrated interrogator includes an ultrasonictransducer array comprising a plurality of ultrasonic transducers. Eachof the ultrasonic transducers in the array is operated by a channel,which includes a switch to alternatively configure the transducer toreceive or transmit ultrasonic waves. FIG. 2B is a schematic of anotherexemplary interrogator for a system described herein. The illustratedinterrogator includes two ultrasonic transducer arrays, with eachultrasonic transducer array including a plurality of ultrasonictransducers. The interrogator also includes an integrated circuit (whichcan include a digital circuit, which can include a processor). Theintegrated circuit is connected to a user interface (which can include adisplay, keyboard, buttons, etc.), a storage medium (i.e., anon-transitory memory), an input/output (which may be wireless, such asa Bluetooth), and a power supply (such as a battery).

FIG. 3A shows a block diagram of an exemplary interrogator that can beworn by a subject. The interrogator includes a wireless communicationsystem (a Bluetooth radio, in the illustration), which can be used tocommunicate with a computer system. FIG. 3B shows an exploded view of awearable interrogator. The interrogator includes a battery, a wirelesscommunication system, and a transducer array. FIG. 3C shows the wearableinterrogator shown in FIG. 3B fully assembled with a harness forattachment to a subject. FIG. 3D illustrates the wearable interrogatorattached a subject, namely a rodent (although could be any other animal,such as a human, dog, cat, horse, cow, pig, sheep, goat, chicken,monkey, rat, or mouse). The interrogator includes a transducer array,which is fixed to the body of the subject by an adhesive.

FIG. 3E illustrates a cross-section of the transducer array of theinterrogator shown in FIGS. 3A-D.

FIG. 4 provides a schematic showing the communication between atransducer from an interrogator and an implantable device having aminiaturized ultrasonic transducer. The interrogator transmitsultrasonic waves to the implantable device, and the miniaturizedultrasonic transducer emits ultrasonic backscatter modulated by thesensor. The backscatter is then received by the interrogator.

FIG. 5A shows a series of cycles of ultrasonic wave pulses emitted by aninterrogator. Upon receiving a trigger from the interrogator (e.g., anFPGA), the transceiver board of the interrogator generates a series oftransmit pulses. At the end of the transmit cycle, the switch on theASIC disconnects the transmit module and connects the receive module.The cycles have a frequency of every 100 microseconds. FIG. 5B shows azoomed-in view of the transmit pulse sequence (i.e., one cycle) shown inFIG. 5A, with the cycle having six pulses of ultrasonic waves at 1.85MHz, the pulses recurring every 540 nanoseconds. FIG. 5C showsultrasonic backscatter emitted by an implantable device. The ultrasonicbackscatter reaches the transducer of the interrogator approximately2t_(Rayleigh). FIG. 5D shows a zoomed-in view of the ultrasonicbackscatter, which can be analyzed. Analysis of the ultrasonicbackscatter can include filtering, rectifying and integrating theultrasonic backscatter waves. FIG. 5E shows a zoomed in view of thefiltered ultrasonic backscatter waves. The backscatter wave includesresponsive regions, which are responsive to changes in impedance to theminiaturized ultrasonic transducer, and non-responsive regions that arenot responsive to changes in impedance to the miniaturized ultrasonictransducer.

FIG. 6 illustrates one embodiment of an implantable device with aminiaturized ultrasonic transducer (identified as the “piezo”) connectedto an ASIC. The ASIC includes a power circuit, a stimulation circuit(which operates the implantable device to emit the stimulatingelectrical pulse), and a modulation circuit (or “backscatter circuit”).The power circuit includes an energy storage capacitor (“cap”). Theelectrodes can be implanted in tissue.

FIG. 7 illustrates an embodiment of an implantable device configured toemit an electrical pulse. The implantable device includes a miniaturizedultrasonic transducer, a power circuit including an energy storagecircuit (which can include one or more capacitors (“cap”), a digitalcircuit, and a pair of electrodes.

FIG. 8A illustrates a schematic of an exemplary implantable deviceincluding a miniaturized ultrasonic transducer and an ASIC on a printedcircuit board (PCB). FIG. 8B illustrates a schematic of anotherexemplary implantable device including a miniaturized ultrasonictransducer and an ASIC on a printed circuit board (PCB).

FIG. 9 illustrates a method of manufacturing an implantable devicedescribed herein.

FIG. 10 is a flowchart for a method of encapsulating an implantabledevice with amorphous silicon carbide.

FIG. 11 shows a closed-loop system for neural recording and stimulation.One or more implantable devices configured to detect anelectrophysiological pulse transmit ultrasonic backscatter to anexternal device (which includes an interrogator). The ultrasonicbackscatter encodes the electrophysiological pulse. The external devicethen transmits ultrasonic waves encoding a trigger signal to one or moreimplantable devices configured to emit an electrical pulse. Upon receiptof the trigger signal, the implantable device emits an electrical pulsethat stimulates the tissue.

FIG. 12 illustrates an implantable device configured to detect anelectrophysiological pulse having a miniaturized ultrasonic transducer,a modulation circuit configured to modulate a current flowing throughthe miniaturized ultrasonic transducer based on an electrophysiologicalsignal detected by an electrode pair.

FIG. 13A illustrates an implantable device configured to detect anelectrophysiological signal with a miniaturized ultrasonic transducer,and integrated circuit, and a pair of electrodes. The integrated circuitincludes a modulation circuit, an AC-coupled amplifier chain, and apower circuit, which includes a full-wave rectifier and doubler, areference, and a regulator. FIG. 13B illustrates an exemplary rectifierthat can be used in the integrated circuit shown in FIG. 13A. FIG. 13Cillustrates an exemplary amplifier chain that can be used in theintegrated circuit shown in FIG. 13A.

FIG. 14A shows different geometries of vias used to connect componentsof the implantable device. FIG. 14B shows a serpentine traceconfiguration for deformable interconnects.

FIG. 15 shows the relationship between time and temperature for curingsilver epoxy, an exemplary material for attaching wirebonds during themanufacture of the implantable device.

FIG. 16 shows a recorded electroneurogram (ENG) using an implantabledevice. The dotted trace shows the signal recorded by the ground truthelectrode. A general profile including the compound action potentialswas reconstructed from the acquired data, which matches the profile ofthe ground truth.

FIG. 17 illustrates a schematic for encapsulating an implantable devicein silicon carbide.

FIG. 18 shows an assembly prototype schematic and PCB.

FIG. 19A-E show processing steps to ensure that the desired miniaturizedultrasonic transducer (PZT) dimension is assembled on the PCB. At FIG.19A, epoxy solder paste is dispensed onto the board. At FIG. 19B, apiezoelectric material is attached to the PCB. At FIG. 16C, thepiezoelectric material is diced to form a bulk piezoelectric ultrasonictransducer of the desired size. At FIG. 19D, the ultrasonic transduceris wirebonded to the PCB. At FIG. 19E, the PCB and ultrasonic transduceris encapsulated in PDMS.

FIG. 20 shows a schematic for measuring electrical impedance with avector network analyzer (VNA),

FIG. 21A shows that the measured power transfer efficiency at variousbulk piezoelectric ultrasonic transducer sizes matches simulatedbehavior. FIG. 21B shows that the measured impedance spectroscopy of aPZT crystal matches a simulation. FIG. 21C shows that the frequencyresponse of harvested power of the miniaturized ultrasonic transducer isapproximately 6.1 MHz.

FIG. 22 is a schematic of an exemplary ultrasonic transducer that can beused as part of an interrogator.

FIG. 23 is a schematic of a setup for acoustic characterization with acalibrated ultrasonic transducer for power delivery verification. Theultrasonic wave receiver is separate from the ultrasonic wavetransmitter.

FIG. 24A shows the output power of a 5 MHz transducer as the hydrophoneis moved away from the transducer's surface. FIG. 24B shows de-ratedpeak is shifted to the left in relation to the water peak.

FIG. 25A shows the XZ cross-section of the transducer output,illustrating a Rayleigh distance and a clear transition from thenear-field to far-field propagation. FIG. 25B shows the XY beamcross-section showing a 6 dB bandwidth of the beam at 2.2 mm.

FIG. 26A shows a focused 2D beam pattern from a transducer array in theXY plane. The measured beam approximates the simulated beam in both theX and Y dimensions. FIG. 26B shows the delay time applied to eachtransducer element in the ultrasonic transducer array.

FIG. 26C shows a simulated 2D XZ cross-sectional beam pattern.

FIG. 27A shows beam steering of an ultrasonic wave beam transmitted froma transducer array. Underneath each beam pattern is the delay for eachtransducer in the array to obtain the measured beam pattern, as shown inFIG. 27B. FIG. 27C shows the 1D beam pattern in the X-axis for each beampattern shown in FIG. 27A. The measured beam pattern closelyapproximates the simulated beam pattern.

FIG. 28 shows a simulated scaling of miniaturized ultrasonic transducerlink efficiency and received power at 5 mm in tissue.

FIGS. 29A-D provide an overview of an exemplary system comprising animplantable device. FIG. 29A shows an external transducer powering andcommunicating an implantable device placed remotely in the body. Drivenby a custom transceiver board, the transducer alternates betweentransmitting a series of pulses that power the device and listening forreflected pulses that are modulated by electrophysiological signals.FIG. 29B shows an implantable device anchored to the sciatic nerve in ananesthetized rat. The insert in FIG. 29B shows an implantable devicewith optional testing leads. FIG. 29C shows components of an exemplaryimplantable device. The implantable device was assembled on a flexiblePCB and included a piezoelectric crystal, a single custom transistor,and a pair of recording electrodes.

FIG. 29D shows a close up of an implantable device on a flexible PCGwith calibration leads to measure electrophysiological signal (groundtruth) and voltages harvested on the piezocrystal. During in-vivoexperiments, the calibration leads were removed.

FIG. 30 illustrates communication between an exemplary interrogator andan implantable device. The top of FIG. 30 is a schematic of the flow ofinformation. The bottom of FIG. 30 represents time traces of signals ateach step referenced in the diagram shown at the top of the figure. AtFIG. 30A, the FPGA from the interrogator generates a trigger signal toinitiate recording. FIG. 30B shows an extracellular,electrophysiological potential presented to the recording electrodes onan implantable device. FIG. 30C shows that upon receiving the triggerfrom the FPGA, the transceiver board generates a series of transmitpulses. At the end of the transmit cycle, the switch on the ASIC of theinterrogator disconnects the transmit module and connects the receivemodule. FIG. 30D shows zoomed-in transmit pulse sequence, showing 6pulses at 1.85 MHz. FIG. 30E shows backscatter from the implantabledevice, which reaches the transducer at approximately 2t_(Rayleigh).FIG. 30F shows zoomed-in backscatter waveforms. The backscatter waveformincludes a large saturating signal which overlaps with the transmittedpulses is electrical feedthrough and is ignored. When returning,backscattered pulses can be seen subsequent to the transmission window.FIG. 30G shows the backscattered waveforms being filtered, rectified,and the area under the curve is computed in order to producereconstructed waveforms. FIG. 30H shows the reconstructed waveformsampled at 10 kHz. Each point of the reconstructed waveform is computedby calculating the area under the curve of the appropriate reflectedpulses, received every 100 μs.

FIG. 31A shows de-rated, normalized peak pressure as a function ofdistance from the surface of an interrogator transducer showed ade-rated focus at ˜8.9 mm at 1.85 MHz. FIG. 31B shows XY cross-sectionalbeampatterns and the corresponding 1-D voltage plot at y=0 atnear-field, Rayleigh distance, and far-field showed beam focusing at theRayleigh distance. FIG. 31C shows that the transducer's output pressurewas a linear function of input voltage (up to 32 V peak-to-peak). FIG.31D (reproduction of FIG. 5E) shows exemplary backscatter waveformshowing different regions of backscatter. The backscatter waveform isfound flanked (in time) by regions which correspond to reflectionsarising from non-responsive regions; these correspond to reflectedpulses from other device components. The measurement from thenonresponsive regions, which do not encode biological data) can be usedas a reference. As a result of taking this differential measurement, anymovements of the entire structure relative to the external transducerduring the experiment can be subtracted out. FIG. 31E shows acalibration curve obtained in the custom water tank setup showed thenoise floor of 0.18 mVrms. FIG. 31F shows the effect of noise floor as afunction of lateral misalignment followed the beampattern powerfall-off. FIG. 31G shows a 1-D plot of the transducer's off-axis voltageand power drop-off at y=0 at Rayleigh distance. FIG. 31H shows a plot ofthe drop in the effective noise floor as a function of angularmisalignment. Angular misalignment results in a skewed beam pattern:ellipsoidal as opposed to circular. This increases the radius of focalspot (spreading energy out over a larger area); the distortion of thefocal spot relaxes the constraint on misalignment.

FIG. 32A shows a in-vivo experimental setup for EMG recording fromgastrocnemius muscle in rats. The implantable device was placed on theexposed muscle surface and the wound was closed with surgical suture.The external transducer couples ultrasound to the implantable device andthe wireless data is recorded and displayed on the computer system(e.g., laptop). FIG. 32B shows a comparison between ground truthmeasurement and the reconstructed EMG signals over a number of trials.20 msec samples were recorded and the inter-stimulus interval was 6 sec.FIG. 32C shows a power spectral density (PSD) of the recorded EMGsignal, which showed 4.29e μV2/Hz and 3.11e μV2/Hz at 107 Hz for groundtruth and the reconstructed dust data, respectively, and severalharmonics due to edges in the waveform. FIG. 32D shows the wirelessbackscatter data recorded at t=0 min and t=30 min matched with R=0.901.

FIG. 33A shows different intensities of EMG signals were recordedin-vivo with the electrodes on the PCB with varying stimulationintensities. FIG. 33B shows similar gradient EMG responses were recordedwirelessly with the implantable device. FIG. 33C shows ground truth andreconstruction of EMG signal from the wireless backscatter data atresponse-saturating stimulation amplitude (100%) matched with R=0.795(R=0.60, 0.64, 0.67, 0.92 for 54%, 69%, 77%, 89%, respectively). In FIG.33D, a quantitative comparison showed <0.4 mV match of the salientfeature. In FIG. 33E, EMG peak-to-peak voltage showed an expectedsigmoidal relationship with the stimulation intensity.

FIG. 34A shows different intensities of ENG signals that were recordedin-vivo with the electrodes on the PCB with varying stimulationintensities. FIG. 34B shows similar gradient ENG responses were recordedwirelessly with the mote. FIG. 34C shows ground truth and reconstructionof ENG signal from the wireless backscatter data at response-saturatingstimulation amplitude (100%) matched with R=0.886 (R=0.822, 0.821, 0.69,0.918, 0.87 for 44%, 61%, 72%, 83%, 89%, respectively). In FIG. 34D,quantitative comparison showed <0.2 mV match of the salient feature. InFIG. 34E, ENG peak-to-peak voltage showed an expected sigmoidalrelationship with the stimulation intensity.

FIG. 35A shows recorded time-domain ENG responses for differentelectrode spacing. FIG. 35B shows peak-to-peak ENG with varyingelectrode spacing.

FIG. 36A shows ultrasonic backscatter from an implantable device, withthe implantable device implanted inn ultrasound coupling gel used tomimic tissue. The backscatter includes a transmit feedthrough andring-down centered at 26 microseconds, and the miniaturized ultrasonictransducer backscatter centered around 47 microseconds. FIG. 36B shows aclose-up on the backscatter region from the miniaturized ultrasonictransducer (the responsive region), which shows amplitude modulation asa result of a signal input to the implantable device.

FIG. 37 shows digital data corresponding to ASCII characters ‘helloworld’ wirelessly ready from the implantable device through pulseamplitude backscatter modulation with unipolar encoding.

DETAILED DESCRIPTION OF THE INVENTION

The implantable device described herein includes a miniaturizedultrasonic transducer (such as a miniaturized piezoelectric transducer)configured to receive ultrasonic waves that power the implantabledevice, a power circuit comprising an energy storage circuit, and two ormore electrodes configured to emit an electrical pulse. The implantabledevice can also include a digital circuit or a mixed-signal integratedcircuit configured to operate the electrodes. The implantable device canbe implanted in a subject such that the electrodes engage a tissue, suchas nervous tissue, muscle tissue, or an organ, and can emit anelectrical pulse to stimulate the tissue. The miniaturized ultrasonictransducer receives ultrasonic energy from an interrogator (which may beexternal or implanted), which powers the implantable device. Theinterrogator includes a transmitter configured to transmit theultrasonic waves to the implantable device. In some embodiments, theinterrogator comprises a receiver, which may be integrated with thetransmitter into a combined transceiver, and the receiver and thetransmitter may be disposed on the same device or on different devices.Mechanical energy from the ultrasonic waves transmitted by theinterrogator vibrates the miniaturized ultrasonic transducer on theimplantable device, which generates an electrical current. Energy fromthe electrical current can be stored in the energy storage circuit,which can include one or more capacitors. The interrogator can encode atrigger signal in the ultrasonic waves that are transmitted to theimplantable device, and, upon receipt of the trigger signal, theimplantable device emits an electrical pulse (for example, bydischarging all or a portion of the energy stored in the energy storagecircuit). The trigger signal can be encoded, for example, at apredetermined signal or in response to some other signal (such as adetected electrophysiological signal in a closed-loop system). Theimplantable device can include a digital circuit, which is configured todecipher the encoded trigger signal, and operate the energy storagecircuit and electrodes to discharge the electrical pulse.

The implantable device or electrodes form the implantable device engagethe tissue to emit the stimulatory electrical pulse. In someembodiments, the tissue is a nervous tissue (such as tissue in thecentral nervous system or peripheral nervous system), muscle tissue(such as smooth muscle, skeletal muscle, or cardiac muscle), or an organ(such as a large or small intestine, stomach, kidney, a secretory gland(such as a salivary gland or mammary gland) or a bladder). In someembodiments, engagement of the tissue is such that the implantabledevice does not completely surround the tissue. In some embodiments, theimplantable device is on, implanted in, or adjacent to the tissue. Insome embodiments, the electrodes of the implantable device engage thetissue. For example, the electrodes can be on or implanted in thenervous tissue (for example, by penetrating the epineurium), muscletissue, or an organ. In some embodiments, the one or more electrodesinclude a cuff electrode, which can partially surround the tissue. Insome embodiments, the implantable device is located near the tissue, andelectrodes can extend from the implantable device to reach the tissue.

The nervous tissue can be part of the central nervous system (such asthe brain (e.g., cerebral cortex, basal ganglia, midbrain, medulla,pons, hypothalamus, thalamus, cerebellum, pallium, or hippocampus) orthe spinal cord), or part of the peripheral nervous system (such as anerve, which may be a somatic nerve or a nerve in the automatic nervoussystem). Exemplary nerves include the sciatic nerve, the vagus nerve,vagus nerve branches, the tibial nerve, the spenic nervie, thesplanchnic nerve, the pudendal nerve, the sacral nerve, the supraorbitalnerve, and the occipital nerve. The muscle tissue can be, for example,skeletal muscle, smooth muscle, or cardiac muscle. Exemplary musclesinclude the gastrocnemius muscle, pelvic floor muscles, gastric smoothmuscle, and cardiac muscle.

The implantable devices described herein can be implanted in or used ina subject (i.e., an animal). In some embodiments, the subject is amammal. Exemplary subjects include a rodent (such as a mouse, rat, orguinea pig), cat, dog, chicken, pig, cow, horse, sheep, rabbit, bird,bat, monkey etc. In some embodiments, the subject is a human.

The electrical pulse can be useful, for example, to control limbs (i.e.,functional electrical stimulation), controlling bladder function, or fortreating sleep apnea or rheumatoid arthritis. See, e.g., Tracey, Theinflammatory reflex, Nature vol. 420, pp. 853-859 (2002).

Generally, in recent years, there has been growing interest in the useof neural recording and stimulation technologies to develop a newclosed-loop neuromodulation therapy paradigm for disorders in thecentral and peripheral nervous systems. Because nerves carry bothefferent and afferent signals to a variety of target organs, effectivetechnologies will need high spatiotemporal resolution to record andstimulate from multiple sites. Additionally, in order for thesetechnologies to become clinically viable, they will need to betetherless to avoid potential infections and adverse biologicalresponses due to externalized leads or micro-motion of the implantwithin the tissue. To address these issues, described herein is anultrasonic backscatter system to wirelessly power and communicate withimplantable devices. One of the strengths of the technology is that,unlike conventional radio frequency technology, ultrasound-based systemsappear scalable down to millimeter size scales, or even smaller, andoperate reliably at greater than several centimeters of implant depth,opening the door to a new technological path in implantable electronics.

In some embodiments, the can be used to record, stimulate, and/or blocksignals (e.g., electrophysiological signals) in the central orperipheral nervous system. Detected electrophysiological signals can beused to trigger and shape the parameters of therapeutic stimulation byproviding detailed feedback about neural dynamics in real time, in thecontext of neurostimulation targets, such as treating sleep apnea orcontrolling bladder function to new disease targets ranging fromdiabetes to rheumatoid arthritis.

Further provided herein are methods to wirelessly power and communicatewith implantable sensors, down to millimeter size scales or smaller,embedded up to several centimeters in tissue to enable continuousmonitoring of body's important vital signs.

The implantable devices described herein can be powered and cancommunicate at depths that were not possible with earlier implantablesystems. In some embodiments, an implantable device configured to detectan electrophysiological single includes a piezoelectric transducer, anapplication specific integrated circuit (ASIC), and a pair of recordingelectrodes. One embodiment of the implant utilizes a single bulkpiezoelectric transducer, either recording or stimulation ASIC, and goldelectrodes. Alternatively, the electrodes can be electroplated orelectrochemically deposited with poly(3,4-ethylenectioxythiophene)(PEDOT), Pt, or Pt-black in order to improve the recording quality.Simultaneous multi-site recording or stimulation can be achieved bydeploying a plurality of these motes at desired locations or placingmultiple pairs of electrodes on the mote and using multiplexors on-chip.The data from different pairs of electrodes can be encoded in eitheramplitude, frequency, or phase modulated waveforms.

In some embodiments, an external unit can interrogate a single mote byemploying a single transducer to transmit ultrasonic energy orinterrogate multiple motes by employing beamforming arrays. The arrayscan be based on an array of bulk piezoelectric transducers or capacitiveor piezoelectric micro-machined ultrasonic transducers (CMUTs, PMUTs).Both PMUTs and CMUTs are micro-electro-mechanical systems (MEMS) devicesmanufactured using semiconductor batch fabrication, with each MUTcapable of transmitting and receiving acoustic waves.

In some embodiments, during usage, the implantable device is placedeither on, around, or in the target nerve with the electrode side incontact with the target nerve. Important connections can be routed out,either a straight or serpentine fashion, to 10 mil vias as test points.The length of the leads can be adjusted according to the application.Alternatively, components can be divided in half and assembled either onthe top or the bottom side of the board, along with the electrodes, inorder to minimize the overall size. Assembly of the double-sidedplatform can be more complex due to the necessary electrical andmechanical isolations between the ASIC and the piezoelectric transducerduring wire-bonding or flip-chip bonding

In some embodiments, the system is implemented as a closed-looptherapeutic medical system. Such a system can include an ultrasoundtransceiver configured to generate and receive ultrasound transmissions,and a body implantable device sized and configured to engage but notentirely surround a neural structure. The implantable device comprises apiezoelectric transducer and energy storage elements to harvest powernecessary to operate stimulation ASIC. The implantable device comprisesstimulation pulse generating circuitry and leads coupled to the pulsegenerating circuitry to produce stimulation pulses to electricallystimulate or block the neural structure. The implantable device furthercomprises an ultrasound backscatter communication system to communicatewith external equipment via the ultrasound transceiver. In someembodiments, the system may include one or more of the following. Theultrasound transceiver may be additionally configured for bodyimplantation. The system may further comprise external equipmentcommunicatively coupled with the ultrasound transceiver. The externalequipment may communicate wirelessly with the ultrasound transceiver.The body implantable device may further be configured to sense abiologic condition, and communicate data indicative of the sensedbiologic condition to the external equipment. In such a case, theexternal equipment is configured to analyze sensed biologic conditiondata, and to initiate, where indicated by the analysis of the sensedbiologic condition data, ultrasound communications to the implanteddevice to generate stimulation pulses to electrically stimulate or blockthe neural structure. In some embodiments, the external equipmentdirectly detects the biological condition.

In some embodiments, the neural stimulation system may include multipleof the body implantable devices sized and configured to engage but notentirely surround a neural structure. The body implantable device mayfurther be configured to communicate data indicative of device state tothe external equipment. The body implantable device may be configured tocommunicate data indicative of device operation to the externalequipment. In addition, the body implantable device may further beconfigured to record and report sensed biologic conditions to providefeedback to adjust stimulation parameters.

A significant advantage of the implantable device described herein isthe ability to emit a stimulatory electrical pulse to nervous tissue ormuscle tissue deep within a subject while being wirelessly powered. Insome embodiments, the implantable device acts in a closed-loop system,and can emit a stimulatory electrical pulse in response to a detectedelectrophysiological pulse. Further, the implantable devices can remainin a subject for an extended period of time without needing to charge abattery or retrieve information stored on the device.

Electromagnetic (EM) power transfer is not a practical for poweringsmall implantable devices due to power attenuation through tissue andthe relatively large apertures (e.g. antennas or coils) required tocapture such energy. See, for example, Seo et al., Neural dust: anultrasonic, low power solution for chronic brain-machine interfaces,arXiv: 1307.2196v1 (Jul. 8, 2013). Use of EM to supply sufficient powerto an implanted device would either require a shallow depth of theimplant or would require excessive heating of the tissue to pass the EMwaves through the tissue to reach the implantable device. In contrast toEM, ultrasonic power transfer provides low power attenuation in tissuedue to the relatively low absorption of ultrasonic energy by tissue andthe shorter wavelength of the ultrasonic waves (as compared toelectromagnetic waves). Further, the shorter wavelengths provided by theultrasonic waves provides high spatial resolution at lower frequenciescompared to radio waves.

Ultrasonic transducers have found application in various disciplinesincluding imaging, high intensity focused ultrasound (HIFU),nondestructive testing of materials, communication, and power deliverythrough steel walls, underwater communications, transcutaneous powerdelivery, and energy harvesting. See, e.g., Ishida et al., InsolePedometer with Piezoelectric Energy Harvester and 2 V Organic Circuits,IEEE J. Solid-State Circuits, vol. 48, no. 1, pp. 255-264 (2013); Wonget al., Advantages of Capacitive Micromachined Ultrasonics Transducers(CMUTs) for High Intensity Focused Ultrasound (HIFU), IEEE UltrasonicsSymposium, pp. 1313-1316 (2007); Ozeri et al., Ultrasonic TranscutaneousEnergy Transfer for Powering Implanted Devices, Ultrasonics, vol. 50,no. 6, pp. 556-566 (2010); and Richards et al., Efficiency of EnergyConversion for Devices Containing a Piezoelectric Component, J.Micromech. Microeng., vol. 14, pp. 717-721 (2004). Unlikeelectromagnetics, using ultrasound as an energy transmission modalitynever entered into widespread consumer application and was oftenoverlooked because the efficiency of electromagnetics for shortdistances and large apertures is superior. However, at the scale of theimplantable devices discussed herein and in tissue, the low acousticvelocity allows operation at dramatically lower frequencies, and theacoustic loss in tissue is generally substantially smaller than theattenuation of electromagnetics in tissue.

The relatively low acoustic velocity of ultrasound results insubstantially reduced wavelength compared to EM. Thus, for the sametransmission distance, ultrasonic systems are much more likely tooperate in the far-field, and hence obtain larger spatial coverage thanan EM transmitter. Further, the acoustic loss in tissue is fundamentallysmaller than the attenuation of electromagnetics in tissue becauseacoustic transmission relies on compression and rarefaction of thetissue rather than time-varying electric/magnetic fields that generatedisplacement currents on the surface of the tissue.

In addition to powering the implantable device, in some embodiments, theultrasonic waves received by the implantable device can include atrigger signal. The trigger signal received by the miniaturizedultrasonic transducer on the implantable device, and is then encoded inthe current generated by the transducer. The signal is then received bya digital signal, which can operate the energy storage circuit torelease an electrical pulse transmitted by the electrode to the tissue.The trigger signal can be transmitted by the ultrasonic waves inresponse to an input signal, such as a user operated input signal, orcan be responsive to a detected electrophysiological signal. Forexample, in some embodiments, the trigger signal is transmitted inresponse to an electrophysiological signal detected by an implantabledevice transmitted to the interrogator.

In some embodiments, a “neural dust” system comprises tiny bodyimplantable devices referred to as neural dust or “motes”, animplantable ultrasound transceiver that communicates with each of themotes using ultrasound transmissions and backscatter transmissionsreflected from the motes, and an external transceiver that communicateswirelessly with the ultrasound transceiver. See Seo et al., Neural dust:an ultrasonic, low power solution for chronic brain-machine interfaces,arXiv: 1307.2196v1 (Jul. 8, 2013) (“Seo et al., 2013”); Seo et al.,Model validation of untethered, ultrasonic neural dust motes forcortical recording, J. Neuroscience Methods, vol. 244, pp. 114-122(2014) (“Seo et al., 2014”); and Bertrand et al., Beamforming approachesfor untethered, ultrasonic neural dust motes for cortical recording: asimulation study, IEE EMBC, vol. 2014, pp. 2625-2628 (2014). The neuraldust system described in these papers is used for cortical recording(i.e., the recording of brain electrical signals). In that applicationas shown in the papers, the motes are implanted in the brain tissue(cortex), the ultrasound transceiver is implanted below the dura, on thecortex, and the external transceiver is placed against the head of thepatient proximate to where the sub-dural ultrasound transceiver isimplanted, as shown in FIG. 1 from Seo et al., 2013.

Seo et al., 2013 and Seo et al., 2014 showed that, theoretically, theneural dust system could be used to develop small-scale implants (belowthe mm-scale) for wireless neural recording. Accurate detection ofelectrophysiological signals or stimulation of tissue using anelectrical pulse is enhanced by accurate determination of the locationor movement of the implantable device. This ensures accurate attributionof a detected signal to the tissue generating the signal, or accuratestimulation of targeted tissue, as well as filtering of signals that maybe caused by movement. As described herein, location and movement of theimplantable devices can be accurately determined by analyzingnon-responsive ultrasonic backscatter. Further, it has been found thatthe implantable devices can transmit a digitized signal encoded in theultrasonic backscatter. The digitized signal can allow for increasedreliability of electrophysiological signal detection (for example, byfiltering false positive signals), data compression (which can beparticularly beneficial, for example, when the implantable deviceincludes a plurality of electrodes), and can allow for the inclusion ofunique identifier signals in the ultrasonic backscatter when utilizing aplurality of implantable devices or when the implantable devices includea plurality of electrodes.

Miniature, implantable systems that exist are either wired, which createproblems for chronic, everyday use, or emerging wireless approachesbased on electromagnetics struggle to power and communicate withimplanted devices at sizes below the millimeter scale or embedded morethan centimeters into the tissue while maintaining power levels withinestablished safety limits. Compared to existing technologies, theproposed implant has the advantage of easy fabrication, integration, andscalability to dimensions and implant depth not achievable in the past.

In some embodiments, an implantable device useful in a close-loop systemstimulates tissue (such as muscle tissue, nervous tissue, or an organ)in response to detected condition (such as an electrophysiologicalsignal, a temperature, a concentration of an analyte (e.g., an ion,glucose, oxygen, etc) or other molecule (such as a neurotransmitter, acytokine, a hormone, or other signaling molecule or protein), pH,pressure, strain, or bioimpedance) detected by the same or a differentimplantable device. The detected condition can be local or systemic. Insome embodiments, the implantable device configured to detect anelectrophysiological signal engages nervous tissue or muscle tissue, andcan be used to report an electroneurogram or an electromyogram. In someembodiments, implantable devices can be configured to detect and report(via ultrasonic backscatter) information related to physiologicalconditions (such as temperature, pressure, pH, or analyte concentration;see International Patent Application titled “IMPLANTS USING ULTRASONICBACKSCATTER FOR SENSING PHYSIOLGOICAL CONDITIONS,” filed on Jul. 7,2017, Attorney Docket No. 416272012040), radiation or radiolabeled cellsand molecules (see International Patent Application titled “IMPLANTSUSING ULTRASONIC BACKSCATTER FOR RADIATION DETECTION AND ONCOLOGY,”filed on Jul. 7, 2017, Attorney Docket No. 416272012140), electricalimpedance of tissue (see International Patent Application titled“IMPLANTS USING ULTRASONIC BACKSCATTER FOR SENSING ELECTRICAL IMPEDANCEOF TISSUE,” filed on Jul. 7, 2017, Attorney Docket No. 416272012440),and an electrophysiological pulse (see International Patent Applicationtitled “IMPLANTS USING ULTRASONIC BACKSCATTER FOR DETECTINGELECTROPHYSIOLOGICAL SIGNALS,” filed on Jul. 7, 2017, Attorney DocketNo. 416272012640); each of these applications is incorporated herein byreference in their entirety for all purposes.

Definitions

As used herein, the singular forms “a,” “an,” and “the” include theplural reference unless the context clearly dictates otherwise.

Reference to “about” a value or parameter herein includes (anddescribes) variations that are directed to that value or parameter perse. For example, description referring to “about X” includes descriptionof “X”.

The term “miniaturized” refers to any material or component about 5millimeters or less (such as about 4 mm or less, about 3 mm or less,about 2 mm or less, about 1 mm or less, or about 0.5 mm or less) inlength in the longest dimension. In certain embodiments, a“miniaturized” material or component has a longest dimension of about0.1 mm to about 5 mm (such as about 0.2 mm to about 5 mm, about 0.5 mmto about 5 mm, about 1 mm to about 5 mm, about 2 mm to about 5 mm, about3 mm to about 5 mm, or about 4 mm to about 5 mm) in length.“Miniaturized” can also refer to any material or component with a volumeof about 5 mm³ or less (such as about 4 mm³ or less, 3 mm³ or less, 2mm³ or less, or 1 mm³ or less). In certain embodiments, a “miniaturized”material or component has a volume of about 0.5 mm³ to about 5 mm³,about 1 mm³ to about 5 mm³, about 2 mm³ to about 5 mm³, about 3 mm³ toabout 5 mm³, or about 4 mm³ to about 5 mm³.

A “piezoelectric transducer” is a type of ultrasonic transceivercomprising piezoelectric material. The piezoelectric material may be acrystal, a ceramic, a polymer, or any other natural or syntheticpiezoelectric material.

A “non-responsive” ultrasonic wave is an ultrasonic wave with areflectivity independent of a detected signal. A “non-responsivereflector” is a component of an implantable device that reflectsultrasonic waves such that the reflected waveform is independent of thedetected signal.

The term “subject” refers to an animal.

It is understood that aspects and variations of the invention describedherein include “consisting” and/or “consisting essentially of” aspectsand variations.

Where a range of values is provided, it is to be understood that eachintervening value between the upper and lower limit of that range, andany other stated or intervening value in that stated range, isencompassed within the scope of the present disclosure. Where the statedrange includes upper or lower limits, ranges excluding either of thoseincluded limits are also included in the present disclosure.

It is to be understood that one, some or all of the properties of thevarious embodiments described herein may be combined to form otherembodiments of the present invention. The section headings used hereinare for organizational purposes only and are not to be construed aslimiting the subject matter described.

Features and preferences described above in relation to “embodiments”are distinct preferences and are not limited only to that particularembodiment; they may be freely combined with features from otherembodiments, where technically feasible, and may form preferredcombinations of features.

The description is presented to enable one of ordinary skill in the artto make and use the invention and is provided in the context of a patentapplication and its requirements. Various modifications to the describedembodiments will be readily apparent to those persons skilled in the artand the generic principles herein may be applied to other embodiments.Thus, the present invention is not intended to be limited to theembodiment shown but is to be accorded the widest scope consistent withthe principles and features described herein. Further, sectionalheadings are provide for organizational purposes and are not to beconsidered limiting. Finally, the entire disclosure of the patents andpublications referred in this application are hereby incorporated hereinby reference for all purposes.

Interrogator

The interrogator can wirelessly communicate with one or more implantabledevices using ultrasonic waves, which are used to power and/or operatethe implantable device. The ultrasonic waves emitted by the interrogatorcan encode a trigger signal, which signals the implantable device toemit an electrical pulse. The interrogator includes one or moreultrasonic transducers, which can operate as an ultrasonic transmitterand/or an ultrasonic receiver (or as a transceiver, which can beconfigured to alternatively transmit or receive the ultrasonic waves).The one or more transducers can be arranged as a transducer array, andthe interrogator can optionally include one or more transducer arrays.In some embodiments, transducers in the array can have regular spacing,irregular spacing, or be sparsely placed. In some embodiments the arrayis flexible. In some embodiments the array is planar, and in someembodiments the array is non-planar. In some embodiments, the ultrasoundtransmitting function is separated from the ultrasound receivingfunction on separate devices. That is, optionally, the interrogatorcomprises a first device that transmits ultrasonic waves to theimplantable device, and a second device that receives ultrasonicbackscatter from the implantable device.

In some embodiments, the interrogator can receive ultrasonic backscatterfrom an implantable device, such an implantable device configured todetect an electrophysiological voltage and emit ultrasonic backscatterwhich encodes information indicative of the detectedelectrophysiological voltage signal. In some embodiments, the triggersignal encoded by the ultrasonic waves emitted from the interrogator andreceived by the implantable device configured to emit an electricalpulse is transmitted in response to a received ultrasonic backscatterencoding information regarding the detected electrophysiological signal.

An exemplary interrogator is shown in FIG. 2A. The illustratedinterrogator shows a transducer array with a plurality of ultrasonictransducers. In some embodiments, the transducer array includes 1 ormore, 2 or more, 3 or more, 5 or more, 7 or more, 10 or more, 15 ormore, 20 or more, 25 or more, 50 or more, 100 or more 250 or more, 500or more, 1000 or more, 2500 or more, 5000 or more, or 10,000 or more ormore transducers. In some embodiments, the transducer array includes100,000 or fewer, 50,000 or fewer, 25,000 or fewer, 10,000 or fewer,5000 or fewer, 2500 or fewer, 1000 or fewer, 500 or fewer, 200 or fewer,150 or fewer, 100 or fewer, 90 or fewer, 80 or fewer, 70 or fewer, 60 orfewer, 50 or fewer, 40 or fewer, 30 or fewer, 25 or fewer, 20 or fewer,15 or fewer, 10 or fewer, 7 or fewer or 5 or fewer transducers. Thetransducer array can be, for example a chip comprising 50 or moreultrasonic transducer pixels. The interrogator shown in FIG. 2Aillustrates a single transducer array; however the interrogator caninclude 1 or more, 2 or more, or 3 or more separate arrays. In someembodiments, the interrogator includes 10 or fewer transducer arrays(such as 9, 8, 7, 6, 5, 4, 3, 2, or 1 transducer arrays). The separatearrays, for example, can be placed at different points of a subject, andcan communicate to the same or different implantable devices. In someembodiments, the arrays are located on opposite sides of an implantabledevice. The interrogator can include an ASIC, which includes a channelfor each transducer in the transducer array. In some embodiments, thechannel includes a switch (indicated in FIG. 2A by “T/Rx”). The switchcan alternatively configure the transducer connected to the channel totransmit ultrasonic waves or receive ultrasonic waves. The switch canisolate the ultrasound receiving circuit from the higher voltageultrasound transmitting circuit. In some embodiments, the transducerconnected to the channel is configured only to receive or only totransmit ultrasonic waves, and the switch is optionally omitted from thechannel. The channel can include a delay control, which operates tocontrol the transmitted ultrasonic waves. The delay control can control,for example, the phase shift, time delay, pulse frequency and/or waveshape (including amplitude and wavelength). The delay control can beconnected to a level shifter, which shifts input pulses from the delaycontrol to a higher voltage used by the transducer to transmit theultrasonic waves. In some embodiments, the data representing the waveshape and frequency for each channel can be stored in a ‘wave table’.This allows the transmit waveform on each channel to be different. Then,delay control and level shifters can be used to ‘stream’ out this datato the actual transmit signals to the transducer array. In someembodiments, the transmit waveform for each channel can be produceddirectly by a high-speed serial output of a microcontroller or otherdigital system and sent to the transducer element through a levelshifter or high-voltage amplifier. In some embodiments, the ASICincludes a charge pump (illustrated in FIG. 2A) to convert a firstvoltage supplied to the ASIC to a higher second voltage, which isapplied to the channel. The channels can be controlled by a controller,such as a digital controller, which operates the delay control. In theultrasound receiving circuit, the received ultrasonic waves areconverted to current by the transducers (set in a receiving mode), whichis transmitted to a data capture circuit. In some embodiments, anamplifier, an analog-to-digital converter (ADC), avariable-gain-amplifier or a time-gain-controlledvariable-gain-amplifier (which can compensate for tissue loss), and/or aband pass filter is included in the receiving circuit. The ASIC can drawpower from a power supply, such as a battery (which is preferred for awearable embodiment of the interrogator). In the embodiment illustratedin FIG. 2A, a 1.8V supply is provided to the ASIC, which is increased bythe charge pump to 32V, although any suitable voltage can be used. Insome embodiments, the interrogator includes a processor and or anon-transitory computer readable memory. In some embodiments, thechannel described above does not include a T/Rx switch but insteadcontains independent Tx (transmit) and Rx (receive) with a high-voltageRx (receiver circuit) in the form of a low noise amplifier with goodsaturation recovery. In some embodiments, the T/Rx circuit includes acirculator. In some embodiments, the transducer array contains moretransducer elements than processing channels in the interrogatortransmit/receive circuitry, with a multiplexer choosing different setsof transmitting elements for each pulse. For example, 64 transmitreceive channels connected via a 3:1 multiplexer to 192 physicaltransducer elements—with only 64 transducer elements active on a givenpulse.

FIG. 2B illustrates another embodiment of interrogator. As shown in FIG.2B, the interrogator includes one or more transducers 202. Eachtransducer 202 is connected to a transmitter/receiver switch 204, whichcan alternatively configure the transducer to transmit or receiveultrasonic waves. The transmitter/receiver switch is connected to aprocessor 206 (such as a central processing unit (CPU), a customdedicated processor ASIC, a field programmable gate array (FPGA),microcontroller unit (MCU), or a graphics processing unit (GPU)). Insome embodiments, the interrogator further includes an analog-digitalconverter (ADC) or digital-to-analog converter (DAC). The interrogatorcan also include a user interface (such as a display, one or morebuttons to control the interrogator, etc.), a memory, a power supply(such as a battery), and/or an input/output port (which may be wired orwireless).

In some embodiments, the interrogator is implantable. An implantedinterrogator may be preferred when the implantable devices are implantedin a region blocked by a barrier that does not easily transmitultrasonic waves. For example, the interrogator can be implantedsubcranially, either subdurally or supradurally. A subcranialinterrogator can communicate with implantable devices that are implantedin the brain. Since ultrasonic waves are impeded by the skull, theimplanted subcranial interrogator allows for communication with theimplantable devices implanted in the brain. In another example, animplantable interrogator can be implanted as part of, behind or withinanother implanted device or prosthetic. In some embodiments, theimplanted interrogator can communicate with and/or is powered by anexternal device, for example by EM or RF signals.

In some embodiments, the interrogator is external (i.e., not implanted).By way of example, the external interrogator can be a wearable, whichmay be fixed to the body by a strap or adhesive. In another example, theexternal interrogator can be a wand, which may be held by a user (suchas a healthcare professional). In some embodiments, the interrogator canbe held to the body via suture, simple surface tension, a clothing-basedfixation device such as a cloth wrap, a sleeve, an elastic band, or bysub-cutaneous fixation. The transducer or transducer array of theinterrogator may be positioned separately from the rest of thetransducer. For example, the transducer array can be fixed to the skinof a subject at a first location (such as proximal to one or moreimplanted devices), and the rest of the interrogator may be located at asecond location, with a wire tethering the transducer or transducerarray to the rest of the interrogator. FIG. 3A-E shows an example of awearable external interrogator. FIG. 3A shows a block diagram of theinterrogator, which includes a transducer array comprising a pluralityof transducers, an ASIC comprising a channel for each transducer in thetransducer array, a battery (lithium polymer (LiPo) battery, in theillustrated example), and a wireless communication system (such as aBluetooth system). FIG. 3B illustrates an exploded view of a wearableinterrogator, including a printed circuit board (PCB) 302, whichincludes the ASIC, a wireless communication system 304, a battery 306,an ultrasonic transducer array 308, and a wire 310 tethering theultrasonic transducer array 308 to the ASIC. FIG. 3C shows the wearableinterrogator 312 shown in FIG. 3B with a harness 314, which can be usedto attach the interrogator to a subject. FIG. 3D shows the assembledinterrogator 316 attached to a subject, with the transducer array 308attached at a first location, and the rest of the interrogator attachedto a second location. FIG. 3E shows a cross-section schematic of anexemplary ultrasonic transducer array 308, which includes a circuitboard 318, vias 320 attaching each transducer 322 to the circuit board318, a metalized polyester film 324, and an absorptive backing layer326. The metalized polyester film 324 can provide a common ground andacoustic matching for the transducers, while the absorptive backinglayer 326 (such as tungsten powder filled polyurethane) can reduceringing of the individual transducers.

The specific design of the transducer array depends on the desiredpenetration depth, aperture size, and the size of the transducers in thearray. The Rayleigh distance, R, of the transducer array is computed as:

${R = {\frac{D^{2} - \lambda^{2}}{4\lambda} \approx \frac{D^{2}}{4\lambda}}},{D^{2}\operatorname{>>}\lambda^{2}}$

where D is the size of the aperture and λ is the wavelength ofultrasound in the propagation medium (i.e., the tissue). As understoodin the art, the Rayleigh distance is the distance at which the beamradiated by the array is fully formed. That is, the pressure filedconverges to a natural focus at the Rayleigh distance in order tomaximize the received power. Therefore, in some embodiments, theimplantable device is approximately the same distance from thetransducer array as the Rayleigh distance.

The individual transducers in a transducer array can be modulated tocontrol the Raleigh distance and the position of the beam of ultrasonicwaves emitted by the transducer array through a process of beamformingor beam steering. Techniques such as linearly constrained minimumvariance (LCMV) beamforming can be used to communicate a plurality ofimplantable devices with an external ultrasonic transceiver. See, forexample, Bertrand et al., Beamforming Approaches for Untethered,Ultrasonic Neural Dust Motes for Cortical Recording: a Simulation Study,IEEE EMBC (August 2014). In some embodiments, beam steering is performedby adjusting the power or phase of the ultrasonic waves emitted by thetransducers in an array.

In some embodiments, the interrogator includes one or more ofinstructions for beam steering ultrasonic waves using one or moretransducers, instructions for determining the relative location of oneor more implantable devices, instructions for monitoring the relativemovement of one or more implantable devices, instructions for recordingthe relative movement of one or more implantable devices, andinstructions for deconvoluting backscatter from a plurality ofimplantable devices.

Communication Between an Implantable Device and an Interrogator

The implantable device and the interrogator wirelessly communicate witheach other using ultrasonic waves. The implantable device receivesultrasonic waves from the interrogator through a miniaturized ultrasonictransducer on the implantable device. Vibrations of the miniaturizedultrasonic transducer on the implantable device generate a voltageacross the electric terminals of the transducer, and current flowsthrough the device, including, if present, an integrated circuit. Thecurrent can be used to charge an energy storage circuit, which can storeenergy to be used to emit an electrical pulse, for example afterreceiving a trigger signal. The trigger signal can be transmitted fromthe interrogator to the implantable device, signaling that an electricalpulse should be emitted. In some embodiments, the trigger signalincludes information regarding the electrical pulse to be emitted, suchas frequency, amplitude, pulse length, or pulse shape (e.g., alternatingcurrent, direct current, or pulse pattern). A digital circuit candecipher the trigger signal and operate the electrodes and electricalstorage circuit to emit the pulse.

In some embodiments, ultrasonic backscatter is emitted from theimplantable device, which can encode information relating to theimplantable device or the electrical pulse emitted by the implantabledevice. For example, the ultrasonic backscatter can encode averification signal, which verifies that electrical pulse was emitted.In some embodiments, an implantable device is configured to detect anelectrophysiological signal, and information regarding the detectedelectrophysiological signal can be transmitted to the interrogator bythe ultrasonic backscatter. To encode signals in the ultrasonicbackscatter, current flowing through the miniaturized ultrasonictransducer is modulated as a function of the encoded information, suchas a detected electrophysiological signal. In some embodiments,modulation of the current can be an analog signal, which may be, forexample, directly modulated by the detected electrophysiological signal.In some embodiments, modulation of the current encodes a digitizedsignal, which may be controlled by a digital circuit in the integratedcircuit. The backscatter is received by an external ultrasonictransceiver (which may be the same or different from the externalultrasonic transceiver that transmitted the initial ultrasonic waves).The information from the electrophysiological signal can thus be encodedby changes in amplitude, frequency, or phase of the backscatteredultrasound waves.

FIG. 4 illustrates an interrogator in communication with an implantabledevice. The external ultrasonic transceiver emits ultrasonic waves(“carrier waves”), which can pass through tissue. The carrier wavescause mechanical vibrations on the miniaturized ultrasonic transducer(e.g., a miniaturized bulk piezoelectric transducer, a PUMT, or a CMUT).A voltage across the miniaturized ultrasonic transducer is generated,which imparts a current flowing through an integrated circuit on theimplantable device. The current flowing through to the miniaturizedultrasonic transducer causes the transducer on the implantable device toemit backscatter ultrasonic waves. In some embodiments, a detectedelectrophysiological signal either directly or indirectly (such asthough an integrated circuit) modulates the current flowing through theminiaturized ultrasonic transducer, the backscatter waves encodeinformation relating to the detected electrophysiological signal. Thebackscatter waves can be detected by the interrogator, and can beanalyzed to recognize the electrophysiological signal detected by theimplantable device.

Communication between the interrogator and the implantable device canuse a pulse-echo method of transmitting and receiving ultrasonic waves.In the pulse-echo method, the interrogator transmits a series ofinterrogation pulses at a predetermined frequency, and then receivesbackscatter echoes from the implanted device. In some embodiments, thepulses are about 200 nanoseconds (ns) to about 1000 ns in length (suchas about 300 ns to about 800 ns in length, about 400 ns to about 600 nsin length, or about 540 ns in length). In some embodiments, the pulsesare about 100 ns or more in length (such as about 150 ns or more, 200 nsor more, 300 ns or more, 400 ns or more, 500 ns or more, 540 ns or more,600 ns or more, 700 ns or more, 800 ns or more, 900 ns or more, 1000 nsor more, 1200 ns or more, or 1500 ns or more in length). In someembodiments, the pulses are about 2000 ns or less in length (such asabout 1500 ns or less, 1200 ns or less, 1000 ns or less, 900 ns or less,800 ns or less, 700 ns or less, 600 ns or less, 500 ns or less, 400 nsor less, 300 ns or less, 200 ns or less, or 150 ns or less in length).In some embodiments, the pulses are separated by a dwell time. In someembodiments, the dwell time is about 100 ns or more in length (such asabout 150 ns or more, 200 ns or more, 300 ns or more, 400 ns or more,500 ns or more, 540 ns or more, 600 ns or more, 700 ns or more, 800 nsor more, 900 ns or more, 1000 ns or more, 1200 ns or more, or 1500 ns ormore in length). In some embodiments, the dwell time is about 2000 ns orless in length (such as about 1500 ns or less, 1200 ns or less, 1000 nsor less, 900 ns or less, 800 ns or less, 700 ns or less, 600 ns or less,500 ns or less, 400 ns or less, 300 ns or less, 200 ns or less, or 150ns or less in length). In some embodiments, the pulses are square,rectangular, triangular, sawtooth, or sinusoidal. In some embodiments,the pulses output can be two-level (GND and POS), three-level (GND, NEG,POS), 5-level, or any other multiple-level (for example, if using 24-bitDAC). In some embodiments, the pulses are continuously transmitted bythe interrogator during operation. In some embodiments, when the pulsesare continuously transmitted by the interrogator a portion of thetransducers on the interrogator are configured to receive ultrasonicwaves and a portion of the transducers on the interrogator areconfigured to transmit ultrasonic waves. Transducers configured toreceive ultrasonic waves and transducers configured to transmitultrasonic waves can be on the same transducer array or on differenttransducer arrays of the interrogator. In some embodiments, a transduceron the interrogator can be configured to alternatively transmit orreceive the ultrasonic waves. For example, a transducer can cyclebetween transmitting one or more pulses and a pause period. Thetransducer is configured to transmit the ultrasonic waves whentransmitting the one or more pulses, and can then switch to a receivingmode during the pause period. In some embodiments, the one or morepulses in the cycle includes about 1 to about 10 pulses (such as about 2to about 8, or about 4 to about 7, or about 6) pulses of ultrasonicwaves in any given cycle. In some embodiments, the one or more pulses inthe cycle includes about 1 or more, 2 or more, 4 or more, 6 or more, 8or more, or 10 or more pulses of ultrasonic waves in any given cycle. Insome embodiments, the one or more pulses in the cycle includes about 20or fewer, about 15 or fewer, about 10 or fewer, about 8 or fewer, orabout 6 or fewer pulses in the cycle. The pulse cycle can be regularlyrepeated, for example every about 50 microseconds (μs) to about 300 μs(such as about every 75 μs to about 200 μs, or every about 100 μs)during operation. In some embodiments, the cycle is reaped every 50 μsor longer, every 100 μs or longer, every 150 μs or longer, every 200 μsor longer, every 250 μs or longer, or every 300 μs or longer. In someembodiments, the cycle is repeated every 300 μs or sooner, every 250 μsor sooner, every 200 μs or sooner, every 150 μs or sooner, or every 100μs or sooner. The cycle frequency can set, for example, based on thedistance between the interrogator and the implantable device and/or thespeed at which the transducer can toggle between the transmitting andreceiving modes.

FIG. 5 illustrates cycled pulse-echo ultrasonic communication betweenthe interrogator and the implantable device. FIG. 5A shows a series ofpulse cycles with a frequency of every 100 microseconds. During thetransmission of the pulses, the transducers in the array are configuredto transmit the ultrasonic waves. After the pulses are transmitted, thetransducers are configured to receive backscattered ultrasonic waves.FIG. 5B shows a zoom-in view of a cycle, which shows six pulses ofultrasonic waves, with a frequency of every 540 nanoseconds.Backscattered ultrasonic waves detected by the interrogator are shown inFIG. 5C, with a zoom-in view of a single pulse shown in FIG. 5D. Asshown in FIG. 5D, the ultrasonic backscatter received from theimplantable device can be analyzed, which may include filtering (forexample, to remove the wave decay) the backscattered waves, rectifyingthe backscattered waves, and integrating the waves to determine the dataencoded by the waves. In some embodiments, the backscatter waves areanalyzed using a machine learning algorithm. FIG. 5E shows a zoomed inversion of the filtered backscattered waves. The backscatter wave shownin FIG. 5E includes four distinct regions corresponding to reflectionsarising from mechanical boundaries: (1) reflection from thebiocompatible material that encapsulates the implantable device; (2)reflection from the top surface of the miniaturized ultrasonictransducer; (3) reflection from the boundary between the printed circuitboard and the miniaturized ultrasonic transducer; and (4) reflectionfrom the back of the printed circuit board. The amplitude of thebackscatter waves reflected from the surface of the miniaturizedtransducer changed as a function of changes in impedance of the currentflowing through the miniaturized ultrasonic transducer, and can bereferred to as the “responsive backscatter” since this region of thebackscatter can encode information transmitted from the ultrasonicdevice to the interrogator. The other regions of the ultrasonicbackscatter can be referred to as “non-responsive backscatter,” and areuseful in determining the position of the implantable device, movementof the implantable device, and/or temperature changes proximal to theimplantable device, as explained below. In some embodiments, the devicefurther comprises a non-responsive reflector. In some embodiments, thenon-responsive reflector is a cube. In some embodiments, thenon-responsive reflector comprises silicon. In some embodiments, thenon-responsive reflector is a surface of rigid material. Thenon-responsive reflector is attached to the implantable device butelectrically isolated, and can reflect ultrasonic waves that are notresponsive to changes in current impedance.

The frequency of the ultrasonic waves transmitted by the transducer canbe set depending on the drive frequency or resonant frequency of theminiaturized ultrasonic transducer on the implantable device. In someembodiments, the miniaturized ultrasonic transducers are broad-banddevices. In some embodiments, the miniaturized ultrasonic transducersare narrow-band. For example, in some embodiments the frequency of thepulses is within about 20% or less, within about 15% or less, withinabout 10% or less, within about 5% or less of the resonant frequency ofthe miniaturized ultrasonic transducer. In some embodiments, the pulsesare set to a frequency about the resonant frequency of the miniaturizedultrasonic transducer. In some embodiments, the frequency of theultrasonic waves is between about 100 kHz and about 100 MHz (such asbetween about 100 kHz and about 200 kHz, between about 200 kHz and about500 kHz, between about 500 kHz and about 1 MHz, between about 1 MHz andabout 5 MHz, between about 5 MHz and about 10 MHz, between about 10 MHzand about 25 MHz, between about 25 MHz and about 50 MHz, or betweenabout 50 MHz and about 100 MHz). In some embodiments, the frequency ofthe ultrasonic waves is about 100 kHz or higher, about 200 kHz orhigher, about 500 kHz or higher, about 1 MHz or higher, about 5 MHz orhigher, about 10 MHz or higher, about 25 MHz or higher, or about 50 MHzor higher. In some embodiments, the frequency of the ultrasonic waves isabout 100 MHz or lower, about 50 MHz or lower, about 25 MHz or lower,about 10 MHz or lower, about 5 MHz or lower, about 1 MHz or lower, about500 kHz or lower, or about 200 kHz or lower. Higher frequency allows fora smaller miniaturized ultrasonic transducer on the implantable device.However, higher frequency also limits the depth of communication betweenthe ultrasonic transducer and the implantable device. In someembodiments, the implantable device and the ultrasonic transducer areseparated by about 0.1 cm to about 15 cm (such as about 0.5 cm to about10 cm, or about 1 cm to about 5 cm). In some embodiments, theimplantable device and the ultrasonic transducer are separated by about0.1 cm or more, about 0.2 cm or more, about 0.5 cm or more, about 1 cmor more, about 2.5 cm or more, about 5 cm or more, about 10 cm or more,or about 15 cm or more. In some embodiments, the implantable device andthe ultrasonic transducer are separated by about 20 cm or less, about 15cm or less, about 10 cm or less, about 5 cm or less, about 2.5 cm orless, about 1 cm or less, or about 0.5 cm or less.

In some embodiments, the backscattered ultrasound is digitized by theimplantable device. For example, the implantable device can include anoscilloscope or analog-to-digital converter (ADC) and/or a memory, whichcan digitally encode information in current (or impedance) fluctuations.The digitized current fluctuations, which can encode information, arereceived by the ultrasonic transducer, which then transmits digitizedacoustic waves. The digitized data can compress the analog data, forexample by using singular value decomposition (SVD) and leastsquares-based compression. In some embodiments, the compression isperformed by a correlator or pattern detection algorithm. Thebackscatter signal may go through a series of non-linear transformation,such as 4^(th) order Butterworth bandpass filter rectificationintegration of backscatter regions to generate a reconstruction datapoint at a single time instance. Such transformations can be done eitherin hardware (i.e., hard-coded) or in software.

In some embodiments, the digitized data can include a unique identifier.The unique identifier can be useful, for example, in a system comprisinga plurality of implantable devices and/or an implantable devicecomprising a plurality of electrode pairs. For example, the uniqueidentifier can identify the implantable device of origin when from aplurality of implantable devices, for example when transmittinginformation from the implantable device (such as a verification signal).In some embodiments, an implantable device comprises a plurality ofelectrode pairs, which may simultaneously or alternatively emit anelectrical pulse by a single implantable device. Different pairs ofelectrodes, for example, can be configured to emit an electrical pulsein different tissues (e.g., different nerves or different muscles) or indifferent regions of the same tissue. The digitized circuit can encode aunique identifier to identify and/or verify which electrode pairsemitted the electrical pulse.

In some embodiments, the digitized signal compresses the size of theanalog signal. The decreased size of the digitized signal can allow formore efficient reporting of information encoded in the ultrasonicbackscatter. By compressing the size of the transmitted informationthrough digitization, potentially overlapping signals can be accuratelytransmitted.

In some embodiments, an interrogator communicates with a plurality ofimplantable devices. This can be performed, for example, usingmultiple-input, multiple output (MIMO) system theory. For example,communication between the interrogator and the plurality of implantabledevices using time division multiplexing, spatial multiplexing, orfrequency multiplexing. In some embodiments, two or more (such as 3, 4,5, 6, 7, 8, 9, 10 or more, 12 or more, about 15 or more, about 20 ormore, about 25 or more, about 50 or more, or about 100 or more)implantable devices communicate with the interrogator. In someembodiments, about 200 or fewer implantable devices (such as about 150or fewer, about 100 or fewer, about 50 or fewer, about 25 or fewer,about 20 or fewer, about 15 or fewer, about 12 or fewer, or about 10 orfewer implantable devices) are in communication with the interrogator.The interrogator can receive a combined backscatter from the pluralityof the implantable devices, which can be deconvoluted, therebyextracting information from each implantable device. In someembodiments, interrogator focuses the ultrasonic waves transmitted froma transducer array to a particular implantable device through beamsteering. The interrogator focuses the transmitted ultrasonic waves to afirst implantable device, receives backscatter from the firstimplantable device, focuses transmitted ultrasonic waves to a secondimplantable device, and receives backscatter from the second implantabledevice. In some embodiments, the interrogator transmits ultrasonic wavesto a plurality of implantable devices, and then receives ultrasonicwaves from the plurality of implantable devices.

In some embodiments, the interrogator is used to determine the locationor velocity of the implantable device. Velocity can be determined, forexample, by determining the position or movement of a device over aperiod of time. The location of the implantable device can be a relativelocation, such as the location relative on the transducers on theinterrogator. Knowledge of the location or movement of the implantabledevice allows for knowledge of the precise location of theelectrophysiological signal detected in the tissue. By determining thelocation of the implantable device and associating the location with thedetected electrophysiological signal, it is possible to characterize ormonitor the tissue at a more localized point. A plurality of transducerson the interrogator, which may be disposed on the same transducer arrayor two or more different transducer arrays, can collect backscatterultrasonic waves from an implantable device. Based on the differencesbetween the backscatter waveform arising from the same implantabledevice and the known location of each transducer, the position of theimplantable device can be determined. This can be done, for example bytriangulation, or by clustering and maximum likelihood. The differencesin the backscatter may be based on responsive backscatter waves,non-responsive backscatter waves, or a combination thereof.

In some embodiments, the interrogator is used to track movement of theimplantable device. Movement of the implantable device that can betracked by the interrogator includes lateral and angular movement. Suchmovement may arise, for example, due to shifting of one or more organssuch as the liver, stomach, small or large intestine, kidney, pancreas,gallbladder, bladder, ovaries, uterus, or spleen, bones, or cartilage(which may be result, for example, of respiration or movement of thesubject), or variations in blood flow (such as due to a pulse). Movementof the implantable device can be tracked, for example, by monitoringchanges in the non-responsive ultrasonic waves. In some embodiments,movement of the implantable device is determined my comparing therelative location of the implantable device at a first time point to therelative location of the implantable device at a second time point. Forexample, as described above, the location of an implantable device canbe determined using a plurality of transducers on the interrogator(which may be on a single array or on two or more arrays). A firstlocation of the implantable device can be determined at a first timepoint, and a second location of the implantable device can be determinedat a second time point, and a movement vector can be determined based onthe first location at the first time point and the second location atthe second time point.

Implantable Device

An implantable device configured to emit an electrical pulse includeincludes a miniaturized ultrasonic transducer (such as a miniaturizedpiezoelectric transducer, a capacitive micro-machined ultrasonictransducer (CMUT), or a piezoelectric micro-machined ultrasonictransducer (PMUT)) configured to receive ultrasonic waves that power theimplantable device, a power circuit comprising an energy storagecircuit, and two or more electrodes configured to engage a tissue andemit an electrical pulse. In some embodiments, the ultrasonic wavesencode a trigger signal. The implantable device is configured to emitthe electrical pulse upon receipt of the trigger signal. In someembodiments, the implantable device includes an integrated circuit. Theintegrated circuit can include, for example, the power circuit and adigital circuit. The digital circuit or a mixed-signal integratedcircuit can operate the power circuit and the electrodes to signalemission of the electric pulse. In some embodiments, for example whenthe implantable device is configured to emit ultrasonic backscatterencoding information, the integrated circuit can include a modulationcircuit, which can be operated by the digital circuit.

The implantable device can engage the tissue to apply an electricalpulse to the tissue. In some embodiments, the electrodes are placedwithin, placed on, placed near, or in electrical communication with thetissue to be stimulated. In some embodiments, the electrodes arepositioned in contact with the tissue. The tissue can be, for example,nervous tissue, muscle tissue, or an organ. For example, the nervoustissue can be central nervous system nervous tissue (such as the brainor the spinal cord), or peripheral nervous system nervous tissue (e.g.,a nerve). The muscle tissue can be, for example, skeletal muscle,cardiac muscle, or smooth muscle. In some embodiments, the electricalpulse stimulates an action potential in the tissue. In some embodiments,the electrical pulse blocks an action potential in a tissue.

In some embodiments, the electrical pulse emitted by the implantabledevice is a direct current pulse or an alternating current pulse. Insome embodiments, the electrical pulse comprises a plurality of pulses,which may be separated by a dwell time. In some embodiments, theelectrical pulse is about 1 microsecond (μs) or longer (such as about 5μs or longer, about 10 μs or longer, about 20 μs or longer, about 50 μsor longer, about 100 μs or longer, about 250 μs or longer, about 500 μsor longer, about 1 millisecond (ms) or longer, about 5 ms or longer,about 10 ms or longer, about 25 ms or longer, about 50 ms or longer,about 100 ms or longer, about 200 ms or longer, or about 500 ms orlonger). In some embodiments, the electrical pulse is about 1000 ms orshorter (such as about 500 ms or shorter, about 200 ms or shorter, about100 ms or shorter, or about 50 ms or shorter, about 25 ms or shorter,about 10 ms or shorter, about 5 ms or shorter, about 1 ms or shorter,about 500 μs or shorter, about 250 μs or shorter, about 100 μs orshorter, about 50 μs or shorter, about 20 μs or shorter, about 10 μs orshorter, or about 5 μs or shorter). In some embodiments, the dwell timeis about 1 microsecond (μs) or longer (such as about 5 μs or longer,about 10 μs or longer, about 20 μs or longer, about 50 μs or longer,about 100 μs or longer, about 250 μs or longer, about 500 μs or longer,about 1 millisecond (ms) or longer, about 5 ms or longer, about 10 ms orlonger, about 25 ms or longer, or about 50 ms or longer). In someembodiments, the dwell time is about 100 ms or shorter (such as about 50ms or shorter, about 25 ms or shorter, about 10 ms or shorter, about 5ms or shorter, about 1 ms or shorter, about 500 μs or shorter, about 250μs or shorter, about 100 μs or shorter, about 50 μs or shorter, about 20μs or shorter, about 10 μs or shorter, or about 5 μs or shorter).

In some embodiments, the electrical pulse is about 1 microamp (μA) ormore (such as about 5 μA or more, about 10 μA or more, about 25 μA ormore, about 50 μA or more, about 100 μA or more, about 250 μA or more,about 500 μA or more, about 1 milliamp (mA) or more, about 5 mA or more,about 10 mA or more, or about 25 mA or more). In some embodiments, theelectrical pulse is about 50 mA or less (such as about 25 mA or less,about 10 mA or less, about 5 mA or less, about 1 mA or less, about 500μA or less, about 250 μA or less, about 100 μA or less, about 50 μA orless, about 25 μA or less, about 10 μA or less, about 5 μA or less, orabout 1 μA or less.

In some embodiments, the electrical pulse has a current frequency ofabout 0.1 Hz or more (such as about 0.5 Hz or more, about 1 Hz or more,about 5 Hz or more, about 10 Hz or more, about 25 Hz or more, about 50Hz or more, about 100 Hz or more, about 200 Hz or more, about 300 Hz ormore, about 400 Hz or more, about 500 Hz or more about 600 Hz or more,about 700 Hz or more, about 800 Hz or more, about 1 kHz or more, about 2kHz or more, or about 5 kHz or more). In some embodiments, theelectrical pulse has a current frequency of about 10 kHz or less (suchas about 5 kHz or less, about 2 kHz or less, about 1 kHz or less, about800 Hz or less, about 700 Hz or less, about 600 Hz or less, about 500 Hzor less, about 400 Hz or less, about 300 Hz or less, about 200 Hz orless, about 100 Hz or less, about 50 Hz or less, about 25 Hz or less,about 10 Hz or less, about 5 Hz or less, about 1 Hz or less, or about0.5 Hz or less).

In some embodiments, the implantable device generates a voltage pulse inthe tissue. In some embodiments, the voltage is about 50 mV or more(such as about 100 mV or more, about 250 mV or more, about 500 mV ormore about 1 V or more, about 2.5 V or more, about 5 V or more, or about10 V or more). In some embodiments, the voltage is about 20 V or less(such as about 15 V or less, about 10 V or less, about 5 V or less,about 2.5 V or less, about 1 V or less, about 500 mV or less, about 250mV or less, or about 100 mV or less).

In some embodiments, the implantable device comprises a plurality ofelectrodes. In some embodiments, the electrodes are paired. Electrodepairs can be formed from two electrodes; thus, an implantable devicewith three electrodes can have three electrode pairs. Theelectrophysiological signal can be detected between the electrodes inthe electrode pairs. In some embodiments, the implantable devicecomprises 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or more, or 15 or more electrodepairs. In some embodiments, the implantable device comprises 2, 3, 5, 6,7, 8, 9, 10 or more electrodes. In some embodiments, the implantabledevice includes an even number of electrodes, and in some embodimentsthe implantable device includes an odd number of electrodes. In someembodiments, the implantable device includes a multiplexer, which canselect the electrodes in the electrode pair to emit the electricalpulse.

Two or more electrodes interface with (or engage) the tissue (e.g.,nervous tissue or muscular tissue). The electrodes need not be linearlydisposed along the tissue. For example, the electrodes may engage anerve along a transverse axis relative to the nerve, which can emit anelectrical pulse in the transverse direction. Two or more electrodes canengage a nerve along the transverse axis at any angle, such as directlyopposite (i.e., 180°), or less than 180° (such as about 170° or less,about 160° or less, about 150° or less, about 140° or less, about 130°or less, about 120° or less, about 110° or less, about 100° or less,about 90° or less, about 80° or less, about 70° or less, about 60° orless, about 50° or less, about 40° or less, or about 30° or less).

In some embodiments, the electrodes in an electrode pair are separatedby about 5 mm or less (such as about 4 mm or less, about 3 mm or less,about 2 mm or less, about 1.5 mm or less, about 1 mm or less, or about0.5 mm or less). In some embodiments, the electrodes in the electrodepair are separated by about 0.5 mm or more (such as about 1 mm or more,about 1.5 mm or more, about 2 mm or more, about 3 mm or more, or about 4or more. In some embodiments, the electrodes are separated by about 0.5mm to about 1 mm, about 1 mm to about 1.5 mm, about 1.5 mm to about 2mm, about 2 mm to about 3 mm, about 3 mm to about 4 mm, or about 4 mm toabout 5 mm.

In some embodiments, the implantable device includes a power circuit,which includes an energy storage circuit. The energy storage circuit caninclude one or more capacitors. Energy from the ultrasonic waves isconverted into a current by the miniaturized ultrasonic transducer, andcan be stored in the energy storage circuit. The energy can be used tooperate the implantable device, such as providing power to the digitalcircuit or one or more amplifiers, or can be used to generate theelectrical pulse used to stimulate the tissue. In some embodiments, thepower circuit further includes, for example, a rectifier and/or a chargepump.

In some embodiments the integrated circuit includes one or more digitalcircuits or mixed-signal integrated circuits, which can include a memoryand one or more circuit blocks or systems for operating the implantabledevice. These systems can include, for example, an onboardmicrocontroller or processor, a finite state machine implementation ordigital circuits capable of executing one or more programs stored on theimplant or provided via ultrasonic communication between interrogatorand implantable device. In some embodiments, the digital circuitincludes an analog-to-digital converter (ADC), which can convert analogsignal encoded in the ultrasonic waves emitted from the interrogator sothat the signal can be processed by the digital circuit. The digitalcircuit can also operate the power circuit, for example to generate theelectrical pulse to stimulate the tissue. In some embodiments, thedigital circuit receives the trigger signal encoded in the ultrasonicwaves transmitted by the interrogator, and operates the power circuit todischarge the electrical pulse in response to the trigger signal.

In some embodiments, the implantable device emits ultrasonic backscatterthat encodes information. The ultrasonic backscatter can be received bythe interrogator, for example, and deciphered to determine the encodedinformation. The information can be encoded using a modulation circuit(or “backscatter circuit”). The modulation circuit can modulate thecurrent flowing through the miniaturized ultrasonic transducer, whichmodulates the ultrasonic backscatter. In some embodiments, themodulation circuit is operated by a digital circuit, which can encode adigitized signal transmitted to the modulation circuit, which transmitsthe digitized signal to the ultrasonic transducer thereby encodingdigitized information in the ultrasonic backscatter. The modulationcircuit (or “backscatter circuit) includes a switch, such as an on/offswitch or a field-effect transistor (FET). An exemplary FET that can beused with some embodiments of the implantable device is ametal-oxide-semiconductor field-effect transistor (MOSFET). Themodulation circuit can alter the impedance of a current flowing throughthe miniaturized ultrasonic transducer, and variation in current flowingthrough the transducer encodes the electrophysiological signal. In someembodiments, information encoded in the ultrasonic backscatter includesa unique identifier for the implantable device. This can be useful, forexample, to ensure the interrogator is in communication with the correctimplantable device when a plurality of implantable devices is implantedin the subject. In some embodiments, the information encoded in theultrasonic backscatter includes a verification signal that verifies anelectrical pulse was emitted by the implantable device. In someembodiments, the information encoded in the ultrasonic backscatterincludes an amount of energy stored or a voltage in the energy storagecircuit (or one or more capacitors in the energy storage circuit). Insome embodiments, the information encoded in the ultrasonic backscatterincludes a detected impedance. Changes in the impedance measurement canidentify scarring tissue or degradation of the electrodes over time.

FIG. 6 illustrates one embodiment of a miniaturized ultrasonictransducer (identified as the “piezo”) connected to an ASIC. The ASICincludes a power circuit and an optional modulation circuit (or“backscatter circuit”). The power circuit includes an energy storagecapacitor (“cap”). Additionally, the implantable device includes astimulation circuit (e.g., a digital circuit), which can operate thepower circuit and the electrodes, which are implanted in or positionedagainst the tissue to be stimulated.

FIG. 7 illustrates an embodiment of an implantable device configured toemit an electrical pulse. The implantable device includes a miniaturizedultrasonic transducer, a power circuit including an energy storagecircuit (which can include one or more capacitors (“cap”), a digitalcircuit or multi-signal integrated circuit, and a pair of electrodes.The ultrasonic transducer is connected to the power circuit, whichallows energy from the ultrasonic waves to be stored in the energystorage circuit. The power circuit is connected to the digital circuitor multi-signal integrated circuit so that the digital circuit ormulti-signal integrated circuit can operate the power circuit. Thedigital circuit or multi-signal integrated circuit is also connected tothe ultrasonic transducer. When a trigger signal is encoded inultrasonic waves received by the ultrasonic transducer, the digitalcircuit or multi-signal integrated circuit can detect the triggersignal. The digital circuit or multi-signal integrated circuit can thenoperate the power circuit to release energy stored in the energycircuit, thereby emitting an electrical pulse using the electrodes.

The implantable devices are miniaturized, which allows for comfortableand long-term implantation while limiting tissue inflammation that isoften associated with implantable devices. The body forms the core ofthe miniaturized implantable device (e.g., the ultrasonic transducer andthe integrated circuit), and the electrodes branch from the body andengage the tissue to deliver an electrical pulse to stimulate thetissue. In some embodiments, the longest dimension of the implantabledevice or the body of the implantable device is about 5 mm or less,about 4 mm or less, about 3 mm or less, about 2 mm or less, about 1 mmor less, about 0.5 mm or less, or about 0.3 mm or less in length. Insome embodiments, the longest dimension of the implantable device orbody of the implantable device is about 0.2 mm or longer, about 0.5 mmor longer, about 1 mm or longer, about 2 mm or longer, or about 3 mm orlonger in the longest dimension of the device. In some embodiments, thelongest dimension of the implantable device or the body of theimplantable device is about 0.2 mm to about 5 mm in length, about 0.3 mmto about 4 mm in length, about 0.5 mm to about 3 mm in length, about 1mm to about 3 mm in length, or about 2 mm in length.

In some embodiments, one or more of the electrodes are on the body ofthe device, for example a pad on the body of the device. In someembodiments, one or more of the electrodes extend from the body of theimplantable device at any desired length, and can be implanted at anydepth within the tissue. In some embodiments, an electrode is about 0.1mm in length or longer, such as about 0.2 mm or longer, about 0.5 mm orlonger, about 1 mm in length or longer, about 5 mm in length or longer,or about 10 mm in length or longer. In some embodiments, the electrodesare about 15 mm or less in length, such as about 10 mm or less, about 5mm or less, about 1 mm or less, or about 0.5 mm or less in length. Insome embodiments, the first electrode is disposed on the body of theimplantable device and the second electrode extends from the body of theimplantable device.

In some embodiments, the implantable device has a volume of about 5 mm³or less (such as about 4 mm³ or less, 3 mm³ or less, 2 mm³ or less, or 1mm³ or less). In certain embodiments, the implantable device has avolume of about 0.5 mm³ to about 5 mm³, about 1 mm³ to about 5 mm³,about 2 mm³ to about 5 mm³, about 3 mm³ to about 5 mm³, or about 4 mm³to about 5 mm³. The small size of the implantable device allows forimplantation of the device using a biopsy needle.

In some embodiments, the implantable device is implanted in a subject.The subject can be for example, a vertebrate animal, such as a mammal.In some embodiments, the subject is a human, dog, cat, horse, cow, pig,sheep, goat, chicken, monkey, rat, or mouse.

In some embodiments, the implantable device or a portion of theimplantable device (such as the miniaturized ultrasonic transducer andthe integrated circuit) is encapsulated by a biocompatible material(such as a biocompatible polymer), for example a copolymer ofN-vinyl-2-pyrrolidinone (NVP) and n-butylmethacrylate (BMA),polydimethylsiloxane (PDMS), parylene, polyimide, silicon nitride,silicon dioxide, silicon carbide, alumina, niobium, or hydroxyapatite.The silicon carbide can be amorphous silicon carbide or crystallinesilicon carbide. The biocompatible material is preferably impermeable towater to avoid damage or interference to electronic circuitry within thedevice. In some embodiments, the implantable device or portion of theimplantable device is encapsulated by a ceramic (for example, alumina ortitania) or a metal (for example, steel or titanium). In someembodiments, the electrodes or a portion of the electrodes are notencapsulated by the biocompatible material.

In some embodiments, the miniaturized ultrasonic transducer and the ASICare disposed on a printed circuit board (PCB). The electrodes canoptionally be disposed on the PCB, or can otherwise be connected to theintegrated circuit. FIGS. 8A and 8B illustrate exemplary configurationsof the implantable device including a PCB. FIG. 8A shows thepiezoelectric transducer 802 and an ASIC 804 disposed on a first side806 of the PCB 808. A first electrode 810 and a second electrode 812 aredisposed on a second side 814 of the PCB 808. FIG. 8B sows thepiezoelectric transducer 814 on a first side 816 of the PCB 818, and theASIC 820 on the second side 822 of the PCB 818. A first electrode 824initiates on the first side 816 of the PCB, and a second electrode 826is initiates on the second side 822 of the PCB 818. The first electrode824 and the second electrode 826 can extend from the PCB 818 to beconfigured to be in electrical connection with each other through thetissue.

The miniaturized ultrasonic transducer of the implantable device can bea micro-machined ultrasonic transducer, such as a capacitivemicro-machined ultrasonic transducer (CMUT) or a piezoelectricmicro-machined ultrasonic transducer (PMUT), or can be a bulkpiezoelectric transducer. Bulk piezoelectric transducers can be anynatural or synthetic material, such as a crystal, ceramic, or polymer.Exemplary bulk piezoelectric transducer materials include bariumtitanate (BaTiO₃), lead zirconate titanate (PZT), zinc oxide (ZO),aluminum nitride (AlN), quartz, berlinite (AlPO₄), topaz, langasite(La₃Ga₅SiO₁₄), gallium orthophosphate (GaPO₄), lithium niobate (LiNbO₃),lithium tantalite (LiTaO₃), potassium niobate (KNbO₃), sodium tungstate(Na₂WO₃), bismuth ferrite (BiFeO₃), polyvinylidene (di)fluoride (PVDF),and lead magnesium niobate-lead titanate (PMN-PT).

In some embodiments, the miniaturized bulk piezoelectric transducer isapproximately cubic (i.e., an aspect ratio of about 1:1:1(length:width:height). In some embodiments, the piezoelectric transduceris plate-like, with an aspect ratio of about 5:5:1 or greater in eitherthe length or width aspect, such as about 7:5:1 or greater, or about10:10:1 or greater. In some embodiments, the miniaturized bulkpiezoelectric transducer is long and narrow, with an aspect ratio ofabout 3:1:1 or greater, and where the longest dimension is aligned tothe direction of propagation of the carrier ultrasound wave. In someembodiments, one dimension of the bulk piezoelectric transducer is equalto one half of the wavelength (λ) corresponding to the drive frequencyor resonant frequency of the transducer. At the resonant frequency, theultrasound wave impinging on either the face of the transducer willundergo a 180° phase shift to reach the opposite phase, causing thelargest displacement between the two faces. In some embodiments, theheight of the piezoelectric transducer is about 10 μm to about 1000 μm(such as about 40 μm to about 400 μm, about 100 μm to about 250 μm,about 250 μm to about 500 μm, or about 500 μm to about 1000 μm). In someembodiments, the height of the piezoelectric transducer is about 5 mm orless (such as about 4 mm or less, about 3 mm or less, about 2 mm orless, about 1 mm or less, about 500 μm or less, about 400 μm or less,250 μm or less, about 100 μm or less, or about 40 μm or less). In someembodiments, the height of the piezoelectric transducer is about 20 μmor more (such as about 40 μm or more, about 100 μm or more, about 250 μmor more, about 400 μm or more, about 500 μm or more, about 1 mm or more,about 2 mm or more, about 3 mm or more, or about 4 mm or more) inlength.

In some embodiments, the ultrasonic transducer has a length of about 5mm or less such as about 4 mm or less, about 3 mm or less, about 2 mm orless, about 1 mm or less, about 500 μm or less, about 400 μm or less,250 μm or less, about 100 μm or less, or about 40 μm or less) in thelongest dimension. In some embodiments, the ultrasonic transducer has alength of about 20 μm or more (such as about 40 μm or more, about 100 μmor more, about 250 μm or more, about 400 μm or more, about 500 μm ormore, about 1 mm or more, about 2 mm or more, about 3 mm or more, orabout 4 mm or more) in the longest dimension.

The miniaturized ultrasonic transducer is connected two electrodes; thefirst electrode is attached to a first face of the transducer and thesecond electrode is attached to a second face of the transducer, whereinthe first face and the second face are opposite sides of the transduceralong one dimension. In some embodiments, the electrodes comprisesilver, gold, platinum, platinum-black, poly(3,4-ethylenedioxythiophene(PEDOT), a conductive polymer (such as conductive PDMS or polyimide), ornickel. In some embodiments, the transducer is operated in shear-modewhere the axis between the metallized faces (i.e., electrodes) of thetransducer are orthogonal to the motion of the transducer.

In some embodiments, the implantable devices are configured to engagewith nervous tissue. In some embodiments, engagement of the nervoustissue does not completely surround the nervous tissue. In someembodiments, the nervous tissue is part of the central nervous system,such as the brain (e.g., cerebral cortex, basal ganglia, midbrain,medulla, pons, hypothalamus, thalamus, cerebellum, pallium, orhippocampus) or spinal cord. In some embodiments, engagement with braintissue includes electrodes that are implanted in the tissue, whereas thebody of the implantable device is located outside of the tissue. In someembodiments, the nervous tissue is part of the peripheral nervoussystem, such as a peripheral nerve. In some embodiments, the implantabledevice is engaged with a muscle, such as skeletal muscle, cardiacmuscle, or smooth muscle. In some embodiments, electrodes from theimplantable device are engaged with the muscle, such as skeletal muscle,smooth muscle, or cardiac muscle.

Manufacture of an Implantable Device

The implantable devices can be manufactured by attaching a miniaturizedultrasonic transducer (such as a bulk piezoelectric transducer, a CMUT,or a PMUT) to a first electrode on a first face of the piezoelectrictransducer, and a second electrode to a second face of the transducer,wherein the first face and the second face are on opposite sides of thetransducer. The first electrode and the second electrode can be attachedto an integrated circuit, which may be disposed on a printed circuitboard (PCB). The integrated circuit includes a power circuit includingan energy storage circuit. In some embodiments, the integrated circuitincludes a digital circuit (or a multi-signal integrated circuit) and/ora modulation circuit. Two or more electrodes are also attached to theintegrated circuit, and are configured to be in electrical connectionwith each other through the tissue. Attachment of the components to thePCB can include, for example, wirebonding, soldering, flip-chip bonding,or gold bump bonding

Certain piezoelectric materials can be commercially obtained, such asmetalized PZT sheets of varying thickness (for example, PSI-5A4E, PiezoSystems, Woburn, Mass., or PZT 841, APC Internationals, Mackeyville,Pa.). In some embodiments, a piezoelectric material sheet is diced intoa desired size, and the diced piezoelectric material is attached to theelectrodes. In some embodiments, the electrodes are attached to thepiezoelectric material sheet, and the piezoelectric material sheet isdiced to the desired size with the electrodes attached to thepiezoelectric material. The piezoelectric material can be diced using adicing saw with a ceramic blade to cut sheets of the piezoelectricmaterial into individualized piezoelectric transducer. In someembodiments, a laser cutter is used to dice or singulate thepiezoelectric material. In some embodiments, patterned etching is usedto dice or singulate the piezoelectric material.

Electrodes can be attached to the top and bottom of the faces of thepiezoelectric transducers, with the distance between the electrodesbeing defined as the height of the piezoelectric transducer. Exemplaryelectrodes can comprise one or more of silver, gold, platinum,platinum-black, poly(3,4-ethylenedioxythiophene (PEDOT), a conductivepolymer (such as conductive PDMS or polyimide), or nickel. In someembodiments, the electrode is attached to the piezoelectric transducerby electroplating or vacuum depositing the electrode material onto theface of the piezoelectric transducer. In some embodiments, theelectrodes are soldered onto the piezoelectric transducer using anappropriate solder and flux. In some embodiments, the electrodes areattached to the piezoelectric transducer using an epoxy (such as asilver epoxy) or low-temperature soldering (such as by use of a solderpaste).

In an exemplary embodiment, solder paste is applied to a pad on aprinted circuit board (PCB), either before or after the integratedcircuit is attached to the PCB. The size of the pad on the circuit boardcan depend on the desired size of the piezoelectric transducer. Solelyby way of example, if the desired size of piezoelectric transducer isabout 100 μm×100 μm×100 μm, the pad can be about 100 μm×100 μm. The padfunctions as the first electrode for the implantable device. Apiezoelectric material (which may be larger than the pad) is placed onthe pad, and is held to the pad by the applied solder paste, resultingin a piezoelectric-PCB assembly. The piezoelectric-PCB assembly isheated to cure the solder paste, thereby bonding the piezoelectrictransducer to the PCB. If the piezoelectric material is larger than thepad, the piezoelectric material is cut to the desired size, for exampleusing a wafer dicing saw or a laser cutter. Non-bonded portions of thepiezoelectric material (for example, the portions of the piezoelectricmaterial that did not overlay the pad) are removed. A second electrodeis attached to the piezoelectric transducer and the PCB, for example byforming a wirebond between the top of the piezoelectric transducer andthe PCB, which completes the circuit. The wirebond is made using a wiremade from any conductive material, such as aluminum, copper, silver, orgold.

The integrated circuit and the miniaturized transducer can be attachedon the same side of the PCB or on opposite sides of the PCB. In someembodiments, the PCB is a flexible PCB, the integrated circuit and theminiaturized transducer are attached to the same side of the PCB, andthe PCB is folded, resulting in an implantable device in which theintegrated circuit and the miniaturized transducer are on opposite sidesof the PCB.

Optionally, the device or a portion of the device is encapsulated in ora portion of the device is encapsulated in a biocompatible material(such as a biocompatible polymer), for example a copolymer ofN-vinyl-2-pyrrolidinone (NVP) and n-butylmethacrylate (BMA),polydimethylsiloxane (PDMS, e.g., Sylgard 184, Dow Corning, Midland,Mich.), parylene, polyimide, silicon nitride, silicon dioxide, alumina,niobium, hydroxyapatite, or silicon carbide. The silicon carbide can beamorphous silicon carbide or crystalline silicon carbide. In someembodiments, the biocompatible material (such as amorphous siliconcarbide) is applied to the device by plasma enhanced chemical vapordeposition (PECVD) or sputtering. PECVD may use precursors such as SiH₄and CH₄ to generate the silicon carbide. In some embodiments, theimplantable device or portion of the implantable device is encased in aceramic (for example, alumina or titania) or a metal (for example, steelor titanium) suitable for medical implantation.

FIG. 9 illustrates an exemplary method of producing the implantabledevice described herein. At step 902, an application specific integratedcircuit (ASIC) is attached to a PCB. The PCB can include two or moreelectrodes for emitting an electrical pulse to stimulate the tissue. Asolder (such as a silver epoxy) can be applied to the PCB (for example,at a first pad disposed on the PCB), and the ASIC can be placed on thesolder. The solder can be cured, for example by heating the PCB with theASIC. In some embodiments, the PCB with the ASIC is heated to about 50°C. to about 200° C., such as about 80° C. to about 170° C., or about150° C. In some embodiments, the PCB with the ASIC is heated for about 5minutes to about 600 minutes, such as about 10 minutes to about 300minutes, about 10 minutes to about 100 minutes, about 10 minutes toabout 60 minutes, about 10 minutes to about 30 minutes, or about 15minutes. Optionally, the ASIC is coated with additional solder. At step904, a piezoelectric transducer (the “piezo” in FIG. 9) is attached tothe PCB. A solder (such as a silver epoxy) can be applied to the PCB(for example, at a second pad disposed on the PCB), and a piezoelectricmaterial can be placed on the solder. The piezoelectric material can bea fully formed (i.e., “diced”) piezoelectric transducer, or can be apiezoelectric material sheet that is cut to form the piezoelectrictransducer once attached to the PCB. The solder can be cured, forexample by heating the PCB with the piezoelectric material. In someembodiments, the PCB with the piezoelectric material is heated to about50° C. to about 200° C., such as about 80° C. to about 170° C., or about150° C. In some embodiments, the PCB with the piezoelectric material isheated for about 5 minutes to about 600 minutes, such as about 10minutes to about 300 minutes, about 10 minutes to about 100 minutes,about 10 minutes to about 60 minutes, about 10 minutes to about 30minutes, or about 15 minutes. The piezoelectric material can be cutusing a saw or laser cutter to the desired dimensions. In someembodiments, the piezoelectric material is a solgel (such as a PZTsolgel) and the transducer material can be shaped with deep reactive ionetching (DRIE). Although FIG. 9 illustrates attachment of the ASIC tothe PCB at step 902 prior to attachment of the piezoelectric material tothe PCB at step 904, a person of skill in the art will appreciate thatthe ASIC and the piezoelectric material can be attached in any order. Atstep 906, the ASIC and the piezoelectric transducer are wirebonded tothe PCB. Although the method illustrated in FIG. 9 shows the ASIC andthe piezoelectric transducer to the PCB after the ASIC and thepiezoelectric transducer are attached to the PCB, a person of skill inthe art will appreciate that the ASIC can be wirebonded to the PCB afterthe ASIC is attached to the PCB, and can be wirebonded either before orafter attachment of the piezoelectric transducer. Similarly, thepiezoelectric transducer may be wirebonded to the PCB either before orafter attachment or wirebonding of the ASIC to the PCB. At step 908, atleast a portion of the device is coated with a biocompatible material.Preferably, at least the piezoelectric transducer and the ASIC arecoated with the biocompatible material. In some embodiments, the sensoris not or at least a portion of the sensor is not coated with thebiocompatible material. For example, in some embodiments, theimplantable device comprises a pair of electrodes which are not coatedwith the biocompatible material, which allows the electrodes tostimulate the tissue with an electrical pulse. In some embodiments, thebiocompatible material is cured, for example by exposure to UV light orby heating.

In some embodiments, the implantable device or a portion of theimplantable device is encapsulated in an amorphous silicon carbide(a-SiC) film. FIG. 10 illustrates a method of manufacturing animplantable device encapsulated in an a-SiC film. At step 1002, apolyimide layer is applied to a smooth surface. At step 1004, an a-SiClayer is applied to the polyimide layer. This can be done, for example,using plasma enhanced chemical vapor deposition (PECVD), using SiH₄ andCH₄ as precursors. At step 1006, one or more ports are etched into thea-SiC layer. In some embodiments, ports are also etched into thepolyimide layer. The ports provide access for portions of theimplantable device that are not encapsulated by the a-SiC, such asportions of a sensor or an electrode that will contact the tissue afterimplant. In some embodiments, etching comprises reactive-ion etching. Atstep 1008, the implantable device is attached to the a-SiC layer. Theimplantable device may be pre-assembled before being attached to thea-SiC layer, or may be built on the a-SiC. In some embodiments, aprinted circuit board (PCB), miniaturized ultrasonic transducer, andsensor are attached to the a-SiC layer. The miniaturized ultrasonictransducer and the sensor need not come in direct contact with the a-SiClayer, as they may be attached to the PCB. Attachment of miniaturizedultrasonic transducer or sensor to the PCB may occur before or afterattachment of the PCB to the a-SiC layer. In some embodiments,attachment of miniaturized ultrasonic transducer or sensor to the PCBcomprises wirebonding the miniaturized ultrasonic transducer or sensorto the PCB. In some embodiments, the sensor includes a portion thatinterfaces with the ports etched into the a-SiC layer. In someembodiments, an ASIC is attached to the PCB, which may occur before orafter attachment of the PCB to the a-SiC layer. At step 1010, an exposedportion of the implantable device is coated with an a-SiC layer. In someembodiments, the exposed portion of the implantable device is coatedwith an a-SiC layer using PECVD. At step 1012, the encapsulatedimplantable device is embossed, thereby releasing the implantable devicefrom the SiC layer.

Closed-Loop Recording and Stimulation Systems

There remains a need for new electrode-based recording technologies thatcan detect abnormalities in physiological signals and be used to updatestimulation parameters in real time. Features of such technologiespreferably include high-density, stable recordings of a large number ofchannels in single nerves, wireless and implantable modules to enablecharacterization of functionally specific neural and electromyographicsignals, and scalable device platforms that can interface with smallnerves of 100 mm diameter or less, as well as specific muscle fibers.Current approaches to recording peripheral nerve activity fall short ofthis goal; for example, known uses of cuff electrodes are limited torecording compound activity from the entire nerve. Single-leadintrafascicular electrodes can record from multiple sites within asingle fascicle but do not enable high-density recording from discretesites in multiple fascicles. Similarly, surface EMG arrays allow forvery-high-density recording but do not capture fine details of deep orsmall muscles. Recently, wireless devices to enable untethered recordingin rodents and nonhuman primates, as well as mm-scale integratedcircuits for neurosensing applications have been developed. See, e.g.,Biederman et al., A 4.78 mm ² fully-integrated neuromodulation SoCcombining 64 acquisition channels with digital compression andsimultaneous dual stimulation, IEEE J. Solid State Circuits, vol. 5, pp.1038-1047 (2015); Denison et al., A 2μW 100 nV/rtHz chpper-stabilizedinstrumentation amplifier for chronic measurement of neural fieldpotentials, IEEE J. Solid State Circuits, vol. 42, pp. 2934-2945 (2007);and Muller et al., A minimally invasive 64-channel wireless uECOoGimplant, IEE J. Solid State Circuits, vol. 50, pp. 344-359 (2015).However, most wireless systems use electromagnetic (EM) energy couplingand communication, which becomes extremely inefficient in systemssmaller than ˜5 mm due to the inefficiency of coupling radio waves atthese scales within tissue. Further miniaturization of wirelesselectronics platforms that can effectively interface with small-diameternerves will require new approaches.

In some embodiments, a wirelessly powered, scalable backscatterultrasonic implantable system, which is used to record, stimulate,and/or block signals in the central and/or peripheral nervous system. Asshown in FIG. 11, the implant is batteryless and embedded near a singleor groups of neurons or implanted into either a nerve or muscle. Asingle or a group of external units (i.e., interrogators) placed outsidepowers and communicates with a single or a group of implants. In oneembodiment, the implant measures and wirelessly reports recordedelectrophysiological signatures back to the source via backscattermodulation. Alternatively, the implant harvests acoustic waves andconverts it to electrical energy to power the application-specificintegrated circuits (ASIC). The ASIC is used to generate a series ofpulses to stimulate target nerves either electrically or acoustically.Existing clinical solutions for neural recording and stimulation arelimited to recording and stimulating from the entire nerve or a largepopulation of neurons and do not enable high-density recording frommultiple discrete sites. Further known clinical solutions are large andcumbersome for long term use.

In some embodiments, the closed-loop system comprises an interrogatorand an implantable device configured to stimulate a tissue in responseto a detected electrophysiological signal. In some embodiments, theimplantable device is configured to detect the electrophysiologicalsignal. In some embodiments, a second implantable device is configuredto detect the electrophysiological signal. The implantable devices canbe deployed in a large, closed loop network. For example, in someembodiments, the closed-loop system includes a plurality of implantabledevices configured to detect an electrophysiological signal and aplurality of implantable devices configured to emit an electrical pulseto stimulate a tissue.

Implantable Devices for Detecting an Electrophysiological Signal

The implantable device configured for detecting an electrophysiologicalsignal includes a miniaturized ultrasonic transducer (such as aminiaturized piezoelectric transducer, a capacitive micro-machinedultrasonic transducer (CMUT), or a piezoelectric micro-machinedultrasonic transducer (PMUT)) configured to emit ultrasonic backscatterencoding a detected electrophysiological signal, a backscatter circuit(i.e., a modulation circuit) configured to modulate a current flowingthrough the miniaturized ultrasonic transducer based on the detectedelectrophysiological signal, and a first electrode and a secondelectrode configured to detect the electrophysiological signal in atissue. In some embodiments, the implantable device includes anintegrated circuit, which can include the modulation circuit, a digitalcircuit (or multi-signal integrated circuit), and/or a power circuit.Ultrasonic backscatter emitted from the miniaturized ultrasonictransducer can encode information related to the detectedelectrophysiological signal, and is received by an interrogator. Theinterrogator can be the same interrogator that is used to operate theimplantable devices configured to emit the electrical pulse thatstimulates tissue, or a different interrogator.

The modulation circuit (or “backscatter circuit) includes a switch, suchas an on/off switch or a field-effect transistor (FET). An exemplary FETthat can be used with some embodiments of the implantable device is ametal-oxide-semiconductor field-effect transistor (MOSFET). Themodulation circuit can alter the impedance of a current flowing throughthe miniaturized ultrasonic transducer, and the variation in currentflowing through the transducer encodes the electrophysiological signal.

FIG. 12 illustrates an exemplary implantable device for recordingelectrophysiological signals. The implantable device includes aminiaturized ultrasonic transducer 1202, a modulation circuit 1204, afirst electrode 1206, and a second electrode 1208. The first electrode1206 and the second electrode 1208 are configured to engage a tissue(e.g., nervous or muscular tissue) to detect an electrophysiologicalsignal. The modulation circuit includes a transistor 1210, whichincludes a drain 1212, source 1214, and a gate 1216. The gate 1216 isconnected to the first electrode 1206. A resistor bridge 1218 comprisinga first resistor 1220 and a second resistor 1222 bridge the drain 1212and the source 1214. The second electrode 1208 is connected to theresistor bridge 1218 between the first resistor 1220 and the secondresistor 1222. The ultrasonic transducer 1202 includes a firsttransducer electrode 1224 and a second transducer electrode 1226. Theultrasonic transducer 1202 can receive ultrasonic carrier waves from aninterrogator, which generates a current through the circuit. Impedanceof the current flowing through the modulation circuit is a function ofthe gate to source voltage, which is shifted by an electrophysiologicalpulse. The modulated current causes an ultrasonic backscatter to beemitted from the transducer 1202, which encodes the electrophysiologicalpulse.

In some embodiments the integrated circuit includes one or more digitalcircuits or multi-signal integrated circuits, which can include a memoryand one or more circuit blocks or systems for operating the implantabledevice. These systems can include, for example an onboardmicrocontroller or processor, a finite state machine implementation ordigital circuits capable of executing one or more programs stored on theimplant or provided via ultrasonic communication between interrogatorand implant. In some embodiments, the digital circuit includes ananalog-to-digital converter (ADC), which can convert analog signal fromthe electrodes configured to detect the electrophysiological pulse intoa digital signal. In some embodiments, the digital circuit includes adigital-to-analog converter (DAC), which converts a digital signal intoan analog signal prior to directing the signal to a modulator. In someembodiments, the digital circuit or multi-signal integrated circuitoperates the modulation circuit (which can also be referred to as a“backscatter circuit”). In some embodiments, the digital circuit ormulti-signal integrated circuit transmits a signal to the modulationcircuit encoding the detected phase-sensitive current and voltage. Insome embodiments, the digital circuit or multi-signal integrated circuitcan operate the modulation circuit (which can also be referred to as the“backscatter circuit”), which connects to the miniaturized ultrasonictransducer. The digital circuit or multi-signal integrated circuit canalso operate one or more amplifiers, which amplifies the currentdirected to the switch.

In some embodiments, the digital circuit encodes a unique identifier adigitized signal comprising the electrophysiological signal. The uniqueidentifier can identify the implantable device of origin of theultrasonic backscatter (for example, in a system with a plurality ofimplantable devices), or may identify which electrodes on theimplantable device detected the electrophysiological signal.

In some embodiments, the digitized circuit compresses the size of theanalog signal. The decreased size of the digitized signal can allow formore efficient reporting of detected electrophysiological signalsencoded in the ultrasonic backscatter. This can be useful, for example,when an implantable device includes a plurality of electrode pairs thatsimultaneously or approximately simultaneously detect anelectrophysiological signal. By compressing the size of theelectrophysiological signal through digitization, potentiallyoverlapping signals can be accurately transmitted.

In some embodiments the integrated circuit filters falseelectrophysiological signals. In some embodiments, the filtering isperformed by the digital circuit. An unfiltered voltage fluctuation cancause changes in the modulated current, which is encoded in theultrasonic backscatter, which can be recorded as a false positive. Tolimit the false positives, current modulation can be filtered, forexample by requiring the electrophysiological signal to be above apredetermined threshold to cause modulation of the current flowingthrough the ultrasonic transducer. In some embodiments, a spike detectoris used to filter false-positive electrophysiological signals.

In some embodiments, the implantable device comprises one or moreamplifiers. The amplifiers can, for example, amplify anelectrophysiological signal. This may occur, for example, prior to thesignal being transmitted to the digital circuit.

In some embodiments, the integrated circuit includes a power circuit,which is configured to power components of the implanted device. Thepower circuit can include, for example, a rectifier, a charge pump,and/or an energy storage capacitor. In some embodiments, the energystorage capacitor is included as a separate component. Ultrasonic wavesthat induce a voltage differential in the miniaturized ultrasonictransducer provide power for the implantable device, which can bemanaged by the power circuit.

In some embodiments, the implantable device comprises a plurality ofelectrode pairs. Electrode pairs can be formed from two electrodes;thus, an implantable device with three electrodes can have threeelectrode pairs. The electrophysiological signal can be detected betweenthe electrodes in the electrode pairs. In some embodiments, theimplantable device comprises 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or more, or15 or more electrode pairs. In some embodiments, the implantable devicecomprises 2, 3, 5, 6, 7, 8, 9, 10 or more electrodes. In someembodiments, tin implantable device includes a multiplexer, which canselect the electrodes in the electrode pair to detect anelectrophysiological signal.

In some embodiments, the electrodes in an electrode pair are separatedby about 5 mm or less (such as about 4 mm or less, about 3 mm or less,about 2 mm or less, about 1.5 mm or less, about 1 mm or less, or about0.5 mm or less). In some embodiments, the electrodes in the electrodepair are separated by about 0.5 mm or more (such as about 1 mm or more,about 1.5 mm or more, about 2 mm or more, about 3 mm or more, or about 4or more. In some embodiments, the electrodes are separated by about 0.5mm to about 1 mm, about 1 mm to about 1.5 mm, about 1.5 mm to about 2mm, about 2 mm to about 3 mm, about 3 mm to about 4 mm, or about 4 mm toabout 5 mm.

FIG. 13A illustrates an implantable device with a miniaturizedultrasonic transducer, an integrated circuit, and a first electrode andsecond electrode. The integrated circuit includes a modulation circuit,which is configured to receive a signal based on a detectedelectrophysiological signal, and modulate a current flowing through theultrasonic transducer based on the received signal. The integratedcircuit further includes an AC-DC power circuit, which includes afull-wave rectifier and doubler, as well as components for referencingor regulating the supplied power. FIG. 13B illustrates an exemplaryrectifier that can be used with the implantable device. FIG. 13Cillustrates exemplary architecture for an AC-coupled amplifier chain.The electrophysiological signal (“V_(neural) ^(”)) is detected using theelectrodes, and is amplified by the amplifier chain before the signal istransmitted to the modulation circuit.

The implantable devices are miniaturized, which allows for comfortableand long-term implantation while limiting tissue inflammation that isoften associated with implantable devices. The body forms the core ofthe miniaturized implantable device (e.g., the ultrasonic transducer andthe integrated circuit), and the electrodes branch from the body andengage the tissue. In some embodiments, the longest dimension of theimplantable device or the body of the implantable device is about 5 mmor less, about 4 mm or less, about 3 mm or less, about 2 mm or less,about 1 mm or less, about 0.5 mm or less, or about 0.3 mm or less inlength. In some embodiments, the longest dimension of the implantabledevice or body of the implantable device is about 0.2 mm or longer,about 0.5 mm or longer, about 1 mm or longer, about 2 mm or longer, orabout 3 mm or longer in the longest dimension of the device. In someembodiments, the longest dimension of the implantable device or the bodyof the implantable device is about 0.2 mm to about 5 mm in length, about0.3 mm to about 4 mm in length, about 0.5 mm to about 3 mm in length,about 1 mm to about 3 mm in length, or about 2 mm in length. Theelectrodes can extend from the device at any desired length, and can beimplanted at any depth within the tissue. In some embodiments, anelectrode is about 0.1 mm in length or longer, such as about 0.2 mm orlonger, about 0.5 mm or longer, about 1 mm in length or longer, about 5mm in length or longer, or about 10 mm in length or longer. In someembodiments, the electrodes are about 15 mm or less in length, such asabout 10 mm or less, about 5 mm or less, about 1 mm or less, or about0.5 mm or less in length.

In some embodiments, the implantable device has a volume of about 5 mm³or less (such as about 4 mm³ or less, 3 mm³ or less, 2 mm³ or less, or 1mm³ or less). In certain embodiments, the implantable device has avolume of about 0.5 mm³ to about 5 mm³, about 1 mm³ to about 5 mm³,about 2 mm³ to about 5 mm³, about 3 mm³ to about 5 mm³, or about 4 mm³to about 5 mm³.

In some embodiments, the implantable device is implanted in a subject.The subject can be for example, a vertebrate animal, such as a mammal.In some embodiments, the subject is a human, dog, cat, horse, cow, pig,sheep, goat, chicken, monkey, rat, or mouse.

In some embodiments, the implantable device or a portion of theimplantable device (such as the miniaturized ultrasonic transducer andthe integrated circuit) is encapsulated by a biocompatible material(such as a biocompatible polymer), for example a copolymer ofN-vinyl-2-pyrrolidinone (NVP) and n-butylmethacrylate (BMA),polydimethylsiloxane (PDMS), parylene, polyimide, silicon nitride,silicon dioxide, silicon carbide, alumina, niobium, or hydroxyapatite.The silicon carbide can be amorphous silicon carbide or crystallinesilicon carbide. The biocompatible material is preferably impermeable towater to avoid damage or interference to electronic circuitry within thedevice. In some embodiments, the implantable device or portion of theimplantable device is encapsulated by a ceramic (for example, alumina ortitania) or a metal (for example, steel or titanium). In someembodiments, the electrodes or a portion of the electrodes are notencapsulated by the biocompatible material.

The miniaturized ultrasonic transducer of the implantable device can bea micro-machined ultrasonic transducer, such as a capacitivemicro-machined ultrasonic transducer (CMUT) or a piezoelectricmicro-machined ultrasonic transducer (PMUT), or can be a bulkpiezoelectric transducer. Bulk piezoelectric transducers can be anynatural or synthetic material, such as a crystal, ceramic, or polymer.Exemplary bulk piezoelectric transducer materials include bariumtitanate (BaTiO₃), lead zirconate titanate (PZT), zinc oxide (ZO),aluminum nitride (AlN), quartz, berlinite (AlPO₄), topaz, langasite(La₃Ga₅SiO₁₄), gallium orthophosphate (GaPO₄), lithium niobate (LiNbO₃),lithium tantalite (LiTaO₃), potassium niobate (KNbO₃), sodium tungstate(Na₂WO₃), bismuth ferrite (BiFeO₃), polyvinylidene (di)fluoride (PVDF),and lead magnesium niobate-lead titanate (PMN-PT).

In some embodiments, the miniaturized bulk piezoelectric transducer isapproximately cubic (i.e., an aspect ratio of about 1:1:1(length:width:height). In some embodiments, the piezoelectric transduceris plate-like, with an aspect ratio of about 5:5:1 or greater in eitherthe length or width aspect, such as about 7:5:1 or greater, or about10:10:1 or greater. The height h of the miniaturized bulk piezoelectrictransducer is defined as the distance along the shortest aspect ratio.The height of the bulk piezoelectric transducer is equal to one half ofthe resonant frequency (λ) of the transducer. At the resonant frequency,the ultrasound wave impinging on either the face of the transducer willundergo a 180° phase shift to reach the opposite phase, causing thelargest displacement between the two faces. In some embodiments, theheight of the piezoelectric transducer is about 10 μm to about 500 μm(such as about 40 to about 400 μm, or about 100 μm to about 250 μm). Insome embodiments, the height of the piezoelectric transducer about 3 mmor less (such as about 2 mm or less, about 1 mm or less, about 500 μm orless, about 400 μm or less, 250 μm or less, about 100 μm or less, orabout 40 or less). In some embodiments, the height of the piezoelectrictransducer is about 20 μm or more (such as about 40 μm or more, about100 μm or more, about 250 μm or more, about 400 or more, about 500 μm ormore, or about 1 mm or more) in length.

The miniaturized ultrasonic transducer is connected two electrodes; thefirst electrode is attached to a first face of the transducer and thesecond electrode is attached to a second face of the transducer, whereinthe first face and the second face are opposite sides of the transduceralong the height dimension. In some embodiments, the electrodes comprisesilver, gold, platinum, platinum-black, poly(3,4-ethylenedioxythiophene(PEDOT), a conductive polymer (such as conductive PDMS or polyimide), acarbon fiber, or nickel.

In some embodiments, the implantable devices are configured to engagewith nervous tissue. In some embodiments, engagement of the nervoustissue does not completely surround the nervous tissue. In someembodiments, the nervous tissue is part of the central nervous system,such as the brain (e.g., cerebral cortex, basal ganglia, midbrain,medulla, pons, hypothalamus, thalamus, cerebellum, pallium, orhippocampus) or spinal cord. In some embodiments, engagement with braintissue includes electrodes that are implanted in the tissue, whereas thebody of the implantable device is located outside of the tissue. In someembodiments, the nervous tissue is part of the peripheral nervoussystem, such as a peripheral nerve.

In some embodiments, the implantable device is used to detect epilepticactivity. See, for example, Mohseni et al., Guest editorial: Closing theloop via advanced neurotechnologies, IEEE Transactions on Neural Systemsand Rehabilitation Engineering, vol. 20, no. 4, pp. 407-409 (2012). Insome embodiments, the implantable device is used to optimize a cochlearimplant. See, for example, Krook-Magnuson et al., Neuroelectronics andbiooptics: Closed-loop technologies in neurological disorders, JAMANeurology, vol. 72, no. 7, pp. 823-829 (2015).

In some embodiments, the implantable device is engaged with a muscle,such as skeletal muscle, smooth muscle or cardiac muscle. In someembodiments, electrodes from the implantable device are engaged with themuscle, such as skeletal muscle, cardiac muscle, or smooth muscle.

EXEMPLARY EMBODIMENTS Embodiment 1

An implantable device, comprising:

an ultrasonic transducer configured to receive ultrasonic waves thatpower the implantable device and encode a trigger signal;

a first electrode and a second electrode configured to be in electricalcommunication with a tissue and emit an electrical pulse to the tissuein response to the trigger signal; and

an integrated circuit comprising an energy storage circuit.

Embodiment 2

The implantable device of embodiment 1, wherein the electrical pulse isa current pulse.

Embodiment 3

The implantable device of embodiment 1, wherein the electrical pulse isa voltage pulse.

Embodiment 4

The implantable device of any one of embodiments 1-3, wherein the firstelectrode and the second electrode are within the tissue or in contactwith the tissue.

Embodiment 5

The implantable device of any one of embodiments 1-4, wherein theintegrated circuit comprises a digital circuit.

Embodiment 6

The implantable device of any one of embodiments 1-5, wherein theintegrated circuit comprises a mixed-signal integrated circuitconfigured to operate the first electrode and the second electrode.

Embodiment 7

The implantable device of any one of embodiments 1-6, wherein theintegrated circuit comprises a power circuit comprising the energystorage circuit.

Embodiment 8

The implantable device of any one of embodiments 1-7, comprising a bodythat comprises the ultrasonic transducer and the integrated circuit,wherein the body is about 5 mm or less in length in the longestdimension.

Embodiment 9

The implantable device of any one of embodiments 1-8, comprising anon-responsive reflector.

Embodiment 10

The implantable device of any one of embodiments 1-9, wherein the tissueis muscle tissue, organ, or nervous tissue.

Embodiment 11

The implantable device of any one of embodiments 1-10, wherein thetissue is part of the peripheral nervous system or the central nervoussystem.

Embodiment 12

The implantable device of any one of embodiments 1-10, wherein thetissue is a skeletal muscle, smooth muscle, or cardiac muscle.

Embodiment 13

The implantable device of any one of embodiments 1-12, comprising threeor more electrodes.

Embodiment 14

The implantable device of any one of embodiments 1-13, wherein theintegrated circuit comprises an analog-to-digital converter (ADC).

Embodiment 15

The implantable device of any one of embodiments 1-14, wherein theimplantable device comprises a modulation circuit configured to modulatea current flowing through the ultrasonic transducer.

Embodiment 16

The implantable device of embodiment 15, wherein the modulated currentencodes information, and the ultrasonic transducer is configured to emitultrasonic waves encoding the information.

Embodiment 17

The implantable device of embodiment 16, wherein the informationcomprises a signal verifying that an electrical pulse was emitted by theimplantable device, a signal indicating an amount of energy stored inthe energy storage circuit, or a detected impedance.

Embodiment 18

The implantable device of any one of embodiments 15-17, wherein theimplantable device comprises a digital circuit configured to operate themodulation circuit.

Embodiment 19

The implantable device of embodiments 18, wherein the digital circuit isconfigured to transmit a digitized signal to the modulation circuit.

Embodiment 20

The implantable device of embodiment 19, wherein the digitized signalcomprises a unique implantable device identifier.

Embodiment 21

The implantable device of any one of embodiments 15-20, wherein themodulation circuit comprising a switch.

Embodiment 22

The implantable device of embodiment 21, wherein the switch comprises afield effect transistor (FET).

Embodiment 23

The implantable device of any one of embodiments 1-22, wherein theultrasonic transducer has a length of about 5 mm or less in the longestdimension.

Embodiment 24

The implantable device of any one of embodiments 1-23, wherein the bodyhas a volume of about 5 mm3 or less.

Embodiment 25

The implantable device of any one of embodiments 1-24, wherein the bodyhas a volume of about 1 mm3 or less.

Embodiment 26

The implantable device of any one of embodiments 1-25, wherein theultrasonic transducer is configured to receive ultrasonic waves from aninterrogator comprising one or more ultrasonic transducers.

Embodiment 27

The implantable device of any one of embodiments 1-26 wherein theultrasonic transducer is a bulk piezoelectric transducer.

Embodiment 28

The implantable device of embodiment 27, wherein the bulk ultrasonictransducer is approximately cubic.

Embodiment 29

The implantable device of any one of embodiments 1-26, wherein theultrasonic transducer is a piezoelectric micro-machined ultrasonictransducer (PMUT) or a capacitive micro-machined ultrasonic transducer(CMUT).

Embodiment 30

The implantable device of any one of embodiments 1-29, wherein theimplantable device is implanted in a subject.

Embodiment 31

The implantable device of embodiment 30, wherein the subject is a human.

Embodiment 32

The implantable device of any one of embodiments 1-31, wherein theimplantable device is at least partially encapsulated by a biocompatiblematerial.

Embodiment 33

The implantable device of embodiment 32, wherein at least a portion ofthe first electrode and the second electrode are not encapsulated by thebiocompatible material.

Embodiment 34

The implantable device of embodiment 32 or 33, wherein the biocompatiblematerial is a copolymer of N-vinyl-2-pyrrolidinone (NVP) andn-butylmethacrylate (BMA), polydimethylsiloxane (PDMS), parylene,polyimide, silicon nitride, silicon dioxide, alumina, niobium,hydroxyapatite, silicon carbide, titania, steel, or titanium.

Embodiment 35

The implantable device of any one of embodiments 32-34, wherein thebiocompatible material is a ceramic or a metal.

Embodiment 36

A system comprising one or more implantable devices according to any oneof embodiments 1-35 and an interrogator comprising one or moreultrasonic transducers configured to transit ultrasonic waves to the oneor more implantable devices, wherein the ultrasonic waves power the oneor more implantable devices.

Embodiment 37

The system of embodiment 36, wherein the ultrasonic waves encode atrigger signal.

Embodiment 38

The system of embodiment 36 or 37, wherein the system comprises aplurality of implantable devices.

Embodiment 39

The system of embodiment 38, wherein the interrogator is configured tobeam steer transmitted ultrasonic waves to alternatively focus thetransmitted ultrasonic waves on a first portion of the plurality ofimplantable devices or focus the transmitted ultrasonic waves on asecond portion of the plurality of implantable devices.

Embodiment 40

The system of embodiment 38, wherein the interrogator is configured tosimultaneously receive ultrasonic backscatter from at least twoimplantable devices.

Embodiment 41

The system of embodiment 38, wherein the interrogator is configured totransit ultrasonic waves to the plurality of implantable devices orreceive ultrasonic backscatter from the plurality of implantable devicesusing time division multiplexing, spatial multiplexing, or frequencymultiplexing.

Embodiment 42

The system according to any one of embodiments 38-41, wherein theinterrogator is configured to be wearable by a subject.

Embodiment 43

A closed-loop system, comprising:

(a) a first device configured to detect a signal;

(b) an interrogator comprising one or more ultrasonic transducersconfigured to receive the ultrasonic backscatter encoding the signal andemit ultrasonic waves encoding a trigger signal; and

(c) a second device configured to emit an electrical pulse in responseto the trigger signal, wherein the second device is implantable,comprising:

-   -   an ultrasonic transducer configured to receive ultrasonic waves        that power the second device and encode a trigger signal;    -   a first electrode and a second electrode configured to be in        electrical communication with a tissue and emit an electrical        pulse to the tissue in response to the trigger signal; and    -   an integrated circuit comprising an energy storage circuit.

Embodiment 44

The system of embodiment 43, wherein the signal is anelectrophysiological pulse, a temperature, a molecule, an ion, pH,pressure, strain, or bioimpedance.

Embodiment 45

The system of embodiment 43 or 44, wherein the first device isimplantable.

Embodiment 46

The system of any one of embodiments 43-45, wherein the first devicecomprises:

a sensor configured to detect the signal;

an integrated circuit comprising a modulation circuit configured tomodulate a current based on the detected signal, and

a first ultrasonic transducer configured to emit an ultrasonicbackscatter encoding the detected signal from the tissue based on themodulated current

Embodiment 47

The system of embodiment 46, wherein the sensor comprises a firstelectrode and a second electrode configured to be in electricalcommunication with a second tissue and detect and electrophysiologicalsignal.

Embodiment 48

The system of embodiment 47, wherein the first tissue and the secondtissue are the same tissue.

Embodiment 49

The system of embodiment 47, wherein the first tissue and the secondtissue are different tissues.

Embodiment 50

The system of any one of embodiments 43-49, wherein the first electrodeand the second electrode of the second device are within the firsttissue or contact the tissue.

Embodiment 51

The system of any one of embodiments 43-50, wherein the integratedcircuit of the second device comprises a digital circuit.

Embodiment 52

The system of any one of embodiments 43-51, wherein the integratedcircuit of the second device comprises a mixed-signal integrated circuitconfigured to operate the first electrode and the second electrode.

Embodiment 53

The system of any one of embodiments 43-52, wherein the integratedcircuit comprises a power circuit comprising the energy storage circuit.

Embodiment 54

The system of any one of embodiments 43-53, wherein the firstimplantable device or the second implantable device comprises a bodythat comprises the ultrasonic transducer and the integrated circuit,wherein the body is about 5 mm or less in length in the longestdimension.

Embodiment 55

The system of any one of embodiments 43-54, wherein the tissue is muscletissue, an organ, or nervous tissue.

Embodiment 56

The system of any one of embodiments 43-55, wherein the first device andthe second device are implanted in a subject.

Embodiment 57

The system of embodiment 56, wherein the subject is a human.

Embodiment 58

A computer system, comprising:

an interrogator comprising one or more ultrasonic transducers;

one or more processors;

non-transitory computer-readable storage medium storing one or moreprograms configured to be executed by the one or more processors, theone or more programs comprising instructions for operating theinterrogator to emit ultrasonic waves encoding a trigger signal thatsignals an implantable device to emit an electrical pulse to a tissue.

Embodiment 59

The computer system of embodiment 58, wherein the interrogator isoperated to emit ultrasonic waves encoding the trigger signal inresponse to a detected physiological signal.

Embodiment 60

The computer system of embodiment 58, wherein the physiological signalcomprises an electrophysiological pulse, a temperature, a molecule, anion, pH, pressure, strain, or bioimpedance.

Embodiment 61

The computer system of embodiment 59 or 60, wherein the one or moreprograms comprise instructions for detecting the physiological signalbased on ultrasonic backscatter encoding the physiological signalemitted from a second implantable device.

Embodiment 62

The computer system of any one of embodiments 59-61, wherein the one ormore programs comprise instructions for determining a location of thefirst implantable device or the second implantable device relative tothe one or more ultrasonic transducers of the interrogator.

Embodiment 63

The computer system of any one of embodiments 59-62, wherein the one ormore programs comprise instructions for detecting movement of the firstimplantable device or the second implantable device.

Embodiment 64

The computer system of embodiment 63, wherein the movement compriseslateral movement.

Embodiment 65

The computer system of embodiment 63 or 64, wherein the movementcomprises angular movement.

Embodiment 66

A method of electrically stimulating a tissue, comprising:

receiving ultrasonic waves at one or more implantable devices;

converting energy from the ultrasonic waves into an electrical currentthat charges an energy storage circuit;

receiving a trigger signal encoded in the ultrasonic waves at the one ormore implantable devices; and

emitting an electrical pulse that stimulates the tissue in response tothe trigger signal.

Embodiment 67

A method of electrically stimulating a tissue, comprising emittingultrasonic waves encoding a trigger signal from an interrogatorcomprising one or more ultrasonic transducers to one or more implantabledevices configured to emit an electrical pulse to the tissue in responseto receiving the trigger signal.

Embodiment 68

The method of embodiment 66 or 67, wherein the trigger signal istransmitted in response to a detected physiological signal.

Embodiment 69

The method of embodiment 69, wherein the physiological signal comprisesan electrophysiological pulse, a temperature, a molecule, an ion, pH,pressure, strain, or bioimpedance.

Embodiment 70

The method of any one of embodiments 66-69, wherein the tissue is amuscle tissue, an organ, or a nervous tissue.

Embodiment 71

The method of any one of embodiments 66-70, comprising implanting theone or more implantable devices in a subject.

Embodiment 72

The method of embodiment 71, wherein the subject is a human.

Embodiment 73

The method of any one of embodiments 66-72, comprising determining alocation of the one or more implantable devices.

Embodiment 74

The method of any one of embodiments 66-73, comprising detecting angularor lateral movement of the one or more implantable devices.

Embodiment 75

A method of stimulating a tissue, comprising:

receiving ultrasonic waves at one or more implantable devices configuredto detect a physiological signal;

converting energy from the ultrasonic waves into an electrical currentthat flows through a modulation circuit;

detecting the physiological signal;

modulating the electrical current based on the detected physiologicalsignal;

transducing the modulated electrical current into an ultrasonicbackscatter that encodes information related to the detectedphysiological signal; and

emitting the ultrasonic backscatter to an interrogator comprising one ormore transducer configured to receive the ultrasonic backscatter;

emitting ultrasonic waves from the interrogator to one or moreimplantable devices configured to emit an electrical pulse to thetissue;

converting energy from the ultrasonic waves emitted from theinterrogator to the one or more implantable devices configured to emitthe electrical pulse into an electrical current that charges an energystorage circuit;

emitting ultrasonic waves encoding a trigger signal from theinterrogator;

receiving the trigger signal at the one or more implantable devicesconfigured to emit the electrical pulse; and

emitting an electrical pulse that stimulates the tissue in response tothe trigger signal.

Embodiment 76

The method of embodiment 75, wherein the physiological signal comprisesan electrophysiological pulse, a temperature, a molecule, an ion, pH,pressure, strain, or bioimpedance.

EXAMPLES Example 1—Manufacture of an Implantable Device

In short form, the assembly steps of the implantable device are asfollows:

1. Attach ASIC to PCB.

2. Wirebond ASIC ports to PCB

3. Attach piezoelectric element to PCB.

4. Wirebond piezoelectric element ports to PCB.

5. Encapsulate full device except for recording electrodes.

The ASIC measures 450 μm by 500 μm by 500 pm and is fabricated by TaiwanSemiconductor Manufacturing Company's 65 nm process. Each chip containstwo transistors with 5 ports each: source, drain, gate, center, andbulk. Each FET uses the same bulk, so either bulk pad can be bonded to,but the transistors differ in that the transistor padded out to the toprow does not contain a resistor bias network whereas the transistorpadded out in the bottom row does. The chip additionally containssmaller pads for electroplating. The same process can be applied toASIC's with more complex circuitry and thus more pads. These pads werenot used in this example. Three versions of the FET were taped out:

Die 1: Long channel FET with threshold voltage: 500 mV

Die 2: Short channel FET with threshold voltage at 500 mV

Die 3: Native FET with threshold voltage at 0 mV

Confirmation of electrical characteristics of these FETs were measuredusing a specially designed CMOS characterization board which containedof a set of pads as wirebonding targets and a second set of pads inwhich wires were soldered to. A sourcemeter (2400 Sourcemeter, KeithleyInstruments, Cleveland, Ohio) was used to supply V_(DS) to the FET andmeasure I_(DS). An adjustable power supply (E3631A, Agilent, SantaClara, Calif.) was used to modulate V_(GS) and the I-V characteristicsof the FETs were obtained. Uncharacteristic IV curves for type 2 dieswere consistently measured, and upon impedance measurement, found thatthe short channel of the die 2s would short out the FET.

The piezoelectric element is lead-zirconium titanate (PZT). It ispurchased as a disc from APC International and diced into. 750 μm×750μm×750 μm cubes using a wafer saw (DAD3240, Disco, Santa Clara, Calif.)with a ceramic blade (PN CX-010-'270-080-H). This mote size was chosenas it maximized power transfer efficiency. For more details, see Seo etal., Neural dust: an ultrasonic, low power solution for chronicbrain-machine interfaces, arXiv: 1307.2196v1 (Jul. 8, 2013).

The implantable device was implanted in the sciatic nerve of aLong-Evans rat. The nerve is a large diameter nerve bundle whichinnervates the hind limb. The nerve is between 1 and 1.4 mm in diameter,and its size and accessibility make it an ideal candidate for deviceimplantation. While several iterations of the implantable device weremade, the following example discusses the development of two versionsimplanted in rat models.

The implantable device substrate integrates the ASIC with thepiezoelectric element and recording electrodes. The first version of theimplantable device used custom-designed PCBs purchased from TheBoardworks (Oakland, Calif.) as a substrate. The PCBs were made of FR-4and were 30 mil (approximately 0.762 mm) in thickness. The dimensions ofthe board were 3 mm×1 mm. This design was the first attempt anintegrated communication and sense platform, so pad size and spacing waschosen to facilitate assembly at the cost of larger size. To conservePCB real-estate, each face of the PCB included pads for either thepiezoelectric element or the ASIC and its respective connections to thePCB. Additionally, two recording pads were placed on the ASIC-face ofthe board. All exposed electrodes were plated with ENIG by TheBoardworks. The pad for the ASIC to sit on was 500 μm by 500 μm, chosento fit the size of the die. The wirebond target pad size was chosen tobe 200 μm by 200 μm and spaced roughly 200 μm away from the edge of thedie in order to give enough clearance for wirebonding (discussed below).Electrode size and spacing varied and were empirically optimized usingfour pairs of electrodes spaced 2 mm, 1.5 m, 1 mm, and 0.5 mm away fromeach other. It was found that electrodes spacing greater than 1.5 mmwere optimal for recording. Minimal signal attenuation was noted between2 mm and 1.5 mm, and signal strength decreased by about 33% between 1.5mm and 1 mm.

In the second iteration of implantable device, three concerns primaryconcerns were addressed: 1) size, 2) ease of wirebonding, 3)implantation/communication. First, to decrease board thickness the FR-4substrate was replaced with a 2 mil (about 50.8 μm) thick polyimideflexible PCB (AltaFlex, Santa Clara, Calif.), as well as thinning theASIC (Grinding and Dicing Services Inc., San Jose, Calif.) to 100 μm. Tofacilitate bonding, the ASIC and PZT coupon were moved to the same side,with only the recording electrodes on the backside of the substrate.While putting the ASIC and PZT coupon on the same side of the board doesimpose a limit on how much the substrate size can be reduced, spacingbetween the electrodes restricted the board length of at least 2 mm. Topush minimization efforts ASIC bonding pads were reduced to 100 μm by100 μm, but the 200 μm spacing between bonding pads and the ASIC itselfhad to be maintained to provide space for wirebonding. The attachmentpads for the PZT coupon was also shrunk and placed closer to the edge ofthe board, with the rationale that the PZT coupon did not have to whollysit on the board, but could hang off it. Additionally, the location ofthe pads relative to the ASIC was also modified to facilitate bonding.In the original design, the bond pad layout surrounding the ASICrequired two wirebonds to cross. This is not impossible, but verydifficult to avoid shorting the pads. Thus, the pad layout was shiftedso that the bonds are relatively straight paths. Finally, during animalexperiments, it was found that alignment of the implantable device wasquite difficult. To combat this, four 1 inch test leads that extendedoff the board were added, two of which connected directly to the sourceand drain of the device to harvest power could be measured and to usethat as an alignment metric. The other two leads connect to the gate andcenter ports in order to obtain a ground truth signal. In order toprevent confusion over which lead belonged to which port, the vias weregiven unique geometries. See FIG. 14A.

There was some fear that the test leads may be easily broken or wouldeasily displace the mote if force was applied on them. Thus, a versionwith serpentine traces was designed. Serpentine traces (FIG. 14B) haveoften been used to enable deformable interconnects, as their structureallows them to “accordion” out. Conceptually, the serpentine tracedesign can be through of a series of cantilevers in series via connectorbeams.

Along with the presented designs, a miniaturized version of theimplantable device using both sides of the substrate was also designedand assembled. In this design, the board measures roughly 1.5 mm by 0.6mm by 1 mm. Due to the miniaturization of the board, a 5 mil silver wire“tail” was attached to the device for recording. This version was nottested in vivo.

The ASIC and PZT coupon were attached to the PCB substrate usingadhesives. There are three majors concerns to choosing an adhesive: 1)the adhesive needs to fix the ASIC and PZT tightly enough that theultrasonic power from wirebonding does not shake the components, 2) dueto the sub-millimeter scales and pitches of the components/substratepads, application of the adhesive was done in a relatively precise way,and 3) the adhesive must be electrically conductive.

The ASIC and diced PZT were originally attached to the PCB substrateusing a low temperature-curing solder paste. Solder paste consists ofpowder metal solder suspended as spheres in flux. When heat is applied,the solder balls begin to melt and fuse together. However, it was foundthat the curing of the solder paste would often result in translating orrotating the PZT coupon or mote during reflow. This presented problemsfor PZT alignment and power harvesting, as well as problems forwirebonding due to the bondpads no longer being appropriately positionedfrom the chip. However, it was found that a two-part silver epoxy, whichsimply consists of silver particles suspended in epoxy was capable ofcuring without repositioning the chip or PZT coupon. Thus, the ASIC anddiced PZT were pasted onto the PCB using a two-part conductive silverepoxy (H20E, Epotek, Billerica, Mass.). The PCBs were then affixed to aglass slide using Kapton tape (Polyimide Film Tape 5413, 3M, St. Paul,Minn.) and put into a convection oven at 150° C. for 15 minutes to curethe epoxy. While higher temperatures could yield faster curing (FIG.15), care was taken to avoid heating the PZT beyond 160° C., half theCurie temperature of the PZT. Heating the PZT any higher runs the riskof depolarizing the PZT. It was found that the 150° C. cure had noeffect on the CMOS performance.

The connections between the top of the PZT and the PCB as well as theASIC and the PCB were made by wirebonding 1 mil Al wire using anultrasonic wedge bonder (740 DB, West Bond, Scotts Valley, Calif.); inthis method of bonding, the Al wire is threaded through the wedge of thebondhead and ultrasonic energy “scrubs” the Al wire against thesubstrate, generating heat through friction. This heat results inwelding the two materials together.

Wirebonding to the ASIC was challenging to avoid shorts due to the sizeof the CMOS pads and the size of the foot of the wirebond. This problemwas accentuated due to the positioning of the wirebonding targets in thefirst version of the implantable device board, which forced the feet oftwo bonds to be placed across the smaller width of the ASIC pad ratherthan the length. While thinner gold wire was available to use forbonding, the difficulty of bonding gold thermosonically with a wedgebonder made it impractical to use gold wires for bonding with thisequipment. Furthermore, in order to effectively wirebond, it isimportant to have a flat and fixed substrate; hence, our original designof having the ASIC and PZT on different sides of the board often causedtrouble during the wirebonding process in our first version ofimplantable boards. Thus, the substrate design choices made in thesecond iteration of the implantable device (moving ASIC and PZT to thesame side, repositioning the pads to provide straight paths to wirebondtargets) greatly improved wirebonding yield.

Finally, because an ultrasonic bonder was used, it was found thatbonding to the PZT resulted in a charge build up would damage the chiponce the PZT was fully bonded to the substrate. To avoid this, thesource and drain test leads of the device were discharged to Earthground directly prior to wirebonding the PZT.

The final step of the implantable device assembly is encapsulation. Thisstep achieves two goals: 1) insulation of the PZT, bondpads, and ASICfrom aqueous environments and 2) protection of the wirebonds between theASIC/PZT coupon and the PCB. At the same time, there must be some methodto either remove or prevent the encapsulant from covering the recordingelectrodes. Additionally, the encapsulant must not impede deviceimplantation. Finally, while it is not crucial, it is of interest tochoose an encapsulant that is optically transparent so that the devicecan be inspected for physical defects if some damage occurred during theencapsulation.

The first encapsulant used was Crystalbond (509′, SPI Supplies, WestChester, Pa.). Crystalbond is an adhesive that is solid at roomtemperature but begins to soften' at 71° C. and melts into a viscousliquid at 121° C. Upon removing heat from the Crystalbond, itre-solidifies within minutes, allowing for good control. To encapsulatethe implantable device, a small flake of Crystalbond was shaved off witha razor and placed directly over the device. The board was then heatedusing a hotplate, first bringing the temperature to around 70° C. whenthe flake would begin to deform and then slowly increasing thetemperature until the Crystalbond became fully liquid. Once the edge ofthe liquid Crystalbond drop expanded past the furthest wirebond but notthe recording pad, the hotplate was turned off and the board was quicklymoved off the plate onto a cooling chuck where the Crystalbond wouldre-solidify.

While Crystal bond was effective, it was found that UV curable epoxidecould give us better selectivity and biocompatibility, as well as rapidcuring. First, a light-curable acrylic (3526, Loctite, Dusseldorf;Germany) was tested, which cures with exposure to ultraviolet light. Asewing needle was used as an applicator to obtain high precision and theepoxy was cured with a 405 nm laser point for 2 minutes. This epoxyworked well, but was not medical-grade and thus not appropriate for abiological implant. Thus, a medical-grade UV curable epoxy (OG116-31,EPO-TEK, Billercia, Mass.) was tried. The epoxy was cured in a UVchamber (Flash, Asiga, Anaheim Hills, Calif.) with 92 mW/cm² at 365 nmfor 5 minutes. While this epoxy was slightly less viscous than theLoctite epoxy, using a sewing needle again as an applicator allowed forselective encapsulation. As an insulator and protection mechanism forthe wirebonds; the epoxy was very effective, but was found to leakduring prolonged submersion in water (˜1 hour). A second medical gradeepoxy which touted stability for up to a year, was considered (301-2,EPO-TEK, Billerica, Mass.), but was found to be not viscous enough andrequired oven-baking for curing. Despite the instability of the UVepoxy, the duration of use was suitable for acute in vivo experiments.

To improve encapsulant stability, parylene-C was also considered as anencapsulation material. Parylene-C is an FDA approved biocompatiblepolymer which is chemically and biologically inert, a good barrier andelectrical insulator, and extremely conformal when vapor deposited).Vapor deposition of Parylene-C is achieved by vaporizing powderParylene-C dimer at temperatures above 150° C. The vapor Parylene-Cdimer is then heated at 690° C. in order for pyrolysis to occur,cleaving the Parylene-C dimer into monomers. The monomer then fills thechamber, which is kept at room temperature. The monomer almostinstantaneously polymerizes once it comes into contact with anysurfaces. For all devices, Paraylene-C was deposited using a parylenedeposition system (SCS Labcoter 2 Parylene Deposition System, SpecialtyCoating Systems, Indianapolis, Ind.) with the parameters shown inTable 1. Note that the table indicates the chamber gauge temperature as135° C. This is distinct from the actual chamber temperature; rather thechamber gauge is simply the vacuum gauge of the process chamber. It isimportant to keep the temperature to at least 135° C. to preventparylene from depositing onto the gauge. For the first batch of FR-4boards, parylene was addressed by selectivity by using Kapton tape tomask off the electrodes. However, it was found that due to the smallpitch between the recording electrodes and the ASIC wirebonding targets,there was not enough surface area for the tape to affix well to theboard and it often slipped off, resulting in coated electrode pads. Inthe second iteration of implantable device, a parylene coat wasattempted using a strategy in which the entire board was coated, thenremove the parylene off the electrodes with a probe tip. In order toassure that parylene was coated onto the entire device, the implantabledevices were suspended in air by damping them between two stacks ofglass slides.

TABLE 1 Parylene-C Deposition Parameters Furnace Temperature 690 deg. C.Chamber Gauge Temperature 135 deg. C. Vaporizer Temperature 175 deg. C.Base Pressure 14 mTorr Operating Pressure 35 mTorr Paralyene-C Mass 5 g

The following provides additional details for manufacturing theimplantable device.

Before beginning to work with the PCBs, ASICs, or PZT coupons, preparetwo sample holders for the dust boards. To do so, simply take two glassslides (3 mm×1 mm×1 mm slides work well) and put a strip of double-sidedtape on the slide lengthwise. The tape will be used to fix the dustmotes in place so that the rest of the steps can be performed. On one ofthe slides, also add a piece of Kapton tape (3M) sticky-side up on topof the double-sided tape. This slide will be the slide used for curingas the high temperature of the cure can cause problems with the adhesiveon the double-sided tape.

Next, mix a small amount of silver paste by weighing out a 1:1 ratio ofpart A and part B in a weigh boat. A large amount of silver-epoxy is notneeded for the assembly process. Shown below is roughly 10 g of epoxy (5g of each part) which is more than enough for three boards, Note thatthe mixed-silver epoxy has a shelf life of two weeks if placed at 4° C.So leftover epoxy can and should be refrigerated when not in use.Additionally, older epoxies (several days to a week) tend to be slightlymore viscous than fresh epoxy which can make application easier,

The substrates come panelized and will need to be removed. Each board isconnected to the panel at several attachment points on the test leadsand vias—these attachment points can be cut using a micro-scalpel(Feather Safety Razor Co., Osaka, Japan). Once the PCB has beensingulated, using carbon-fiber tipped tweezers or ESD plastic tweezers,place the singulated PCB onto the high-temperature sample holder.

The diced/thinned die are shipped on dicing tape, which can make ittricky to remove the die. In order to reduce the adhesion between thedie and tape, it can be helpful to deform the tape. Using carbon-tippedor ESD plastic tweezers, gently press the tape and work the tweezers ina circular motion around the die. To check if the die has been freed,gently nudge the chip with the tip of the tweezers. If the die does notcome off easily, continue to press into tape surrounding the chip. Oncethe chip has come off, carefully place the chip onto thehigh-temperature sample holder next to its board. It is advisable tobring the sample holder to the chip rather than the other way around sothat the chip is not in transit, Care must be taken in this step toavoid losing or damaging the die. Never force a die off the tape, asexcessive force can cause a chip to fly off the tape.

Next, attach the die using silver epoxy. Under a microscope, use a pinor something equally fine to apply a small amount silver epoxy to theCMOS pad on the PCB. In this step, it is better to err on the side oftoo little epoxy than too much epoxy since more silver paste can alwaysbe applied, but removing silver paste is non-trivial. Small amounts ofuncured epoxy can be scraped away with the same tool used forapplication, just ensure the epoxy has been wiped off the tool.

Once the epoxy has been placed on the pad, the ASIC can be placed ontothe epoxy. Due to a CAD error, some of the chips have been reflected. Itis important to take care that chips which are reflected have beenoriented the correct way on the board to ensure no wires need to crossduring wirebonding.

Once the ASICs have been situated on the boards correctly, the silverepoxy can be cured by placing it into an oven at 150° C. for 15 minutes.Note that different temperatures can be used if needed—see FIG. 15 fordetails. After the silver epoxy has been cured, double-check adhesion bygently pushing on each die, If the die moves; a second coat of silverepoxy will be needed.

To prepare for wirebonding, move the devices from the high-temperaturesample holder to the regular sample holder. This change is necessarybecause the adhesion of double-sided tape is stronger than that of theKapton tape so wirebonding will be made easier. A piece of double-sidedtape should be good enough to affix the sample holder to thewirebonder's workholder. It is best to ensure that the workholder hasnot been previously covered with double-sided tape so that the testleads do not get accidentally stuck to anything. If necessary,clean-room tape can be used to provide additional clamping of the sampleholder.

Ensure the wirebonder is in good condition by making bonds on theprovided test-substrate using default settings. Ensuring that thewirebonder is in condition is important, as a damaged wedge will notbond well and effectively just damage the ASIC pads. Forward bonds(first bond on the die, second bond on the substrate) should be made inthe following order: 1. Gate. 2. Bulk. 3. Center. 4. Drain. 5. Source.While it is not critical that the bonds be made in this order, thisorder minimizes the number of substrate reorientations and preventsaccidental damage to the bonds due to the bondhead. Small angleadjustments of the workholder can be made to facilitate bonding; it isimperative that this bond be as straight as possible. In the case thatthe foot of the second bond lifts from the substrate, changing thenumber of bonds to one and bonding the foot again may help. If properadhesion cannot be made, a potential solution is to connect the foot ofthe bond and the substrate using silver epoxy. Additionally, shortscaused by two bond-feet touching can be resolved by very carefullycutting away the bridging metal using a microscalpel.

Known working bonding parameters can be found in Table 2, below. Theseparameters are simply guidelines and should be modified as necessary.Needing excess power (greater than 490) is typically indicative of aproblem: substrate fixing (both PCB to glass slide and CMOS to PCB),wedge condition, and pad condition should all be checked. In the case ofpad condition, damaged pads due to previous wirebonding attempts willusually require higher power—in some cases, the devices are salvageable,but failed attempts to bond with power higher than 600 usually resultsin too much damage to the pads for good bonding.

TABLE 2 Westbond 7400B A1 Parameters for ASIC Bond # Power Time 1 (ASIC)420 40 ms 2 (Substrate) 420 40 ms

Post-wire bonding, the device should undergo electrical testing toensure proper bonding. If using a type 1 die, the I-V characteristicsshould be roughly as shown in Table 3.

TABLE 3 Typical I-V characteristics for Type 1 Die under V_(ds) = 0.1 VV_(gs) I_(ds)   0 V  0.5 μA 0.1 V 0.74 μA 0.2 V 10.6 μA 0.3 V 51.4 μA0.4 V 0.192 mA  0.5 V 0.39 mA 0.6 V 1.14 mA 0.7 V 1.55 mA 0.8 V 1.85 mA

If the I-V characteristics do not seem correct, a valuabletroubleshooting method is checking the resistances between the drain andcenter, source and center, and drain and source. If the die is workingproperly, one should expect roughly 90 kΩ resistance between the drainand center and source and center, and roughly 180 k Ω between the drainand source.

After confirmation that the FET is connected properly, the PZT couponshould be attached. This is done in a similar fashion to attaching theASIC: place a dab of silver epoxy using a sewing needle on theappropriate pad. It is best to put the epoxy dab on the back edge of thepad (towards the end of the board) since the PZT coupon will not becentered on the pad, but pushed back so that the coupon hangs off theboard. Keep in mind that the polarity of the PZT coupon has a smalleffect on its efficiency. To determine whether or not the coupon is inthe correct position, check if the bottom face is larger than the topface. Due to the path of the dicing saw, the bottom of the coupon, isslightly larger than the top of the coupon. Thus, the edges of thebottom face can be seen from a top down view, then the coupon has beenplaced in the same orientation as it was when the disk was diced.

Wirebonding the PZT is done in a similar manner to the ASIC (forwardbonding, the PZT to the PCB). However, one crucial change is that thedrain and source vias should be grounded. There is an earth ground portnext to Westbond which can be accessed via a banana connector. As aguideline, the parameters shown in Table 4 have been known to work.

TABLE 4 Westbond 7400B A1 Parameters for PZT Bond # Power Time 1 (PZT)390 40 ms 2 (Substrate) 490 40 ms

A successful bond may require several attempts depending on how well thePZT coupon is attached to the substrate. The more attempts that aremade, the worse the mechanical structure of the PZT becomes (the silvercoating will become damaged) so it is best to try to very quicklyoptimize the process. Bonds that fail due to foot detachment generallyimply not enough power. Bonds that fail due to the wire breaking at thefoot generally imply too much power.

After a successful bond is made, it is always good to do anotherelectrical test to ensure that bonding the PZT has not damaged the ASIC.

As a final step, test wires were soldered to the vias and encapsulatethe device, The test wires are 3 mil silver wires. Nate that these wiresare insulated: the insulation can be removed by putting the wire closeto a flame (not in the flame) and watching the plastic melt and recede.

After soldering wires, the device can now be encapsulated. Theencapsulant is OG116-31 medical-grade UV curable epoxy and should bedispensed using a sewing needle. An effective method is to put a largedrop of epoxy over the PZT coupon and a large drop over the ASIC. Usinga clean needle, push the droplet over the board so that the entiretopside of the board is coated. The epoxy should wet the board, but notspill over due to its surface tension. Once the main body of the boardis coated, the vias should also be coated, as well as the side faces ofthe piezo. The board can then be cured in a UV chamber for roughly 5minutes. It has been found that the test wires can occasionally contactsomething in the UV chamber and short the ASIC. Thus, prior to puttingthe board in the chamber, it is good to wrap the wires down or place iton some tape in order to isolate them from any chamber surfaces.

Following curing, the backside should be coated. In particular theexposed PZT coupon which hangs over the board as well as the backside ofthe test vias and the two vias on the backside of the board whichconnect the electrodes to the topside of the board. This part can be alittle tricky due to the small space between the backside vias and theelectrodes, so it is best to start with a very small amount of epoxy andplace it near the edge of the board, then drag the epoxy up towards thevias. The backside of the board should be cured in the same manner asthe topside. Once the board is fully encapsulated, a final electricaltest should be done, and upon passing, the implantable device is nowcomplete.

Example 2—Set-Up for Testing Implantable Devices

Testing of implantable has always been tricky due to the thinness of thetest leads that extend out from the board. Clipping onto and off ofthese vias for I-V measurements has often resulted in pulling the leadsoff the body of the device. Furthermore, due to the test leads, it isdifficult to perform water-tank test measurements; as exposedelectronics in water would result in shorts. In order to circumvent thisissue, a PCB was designed to serve as a testbed for implantable devicemeasurements. The PCB (Bay Area Circuits, Fremont, Calif.) was made ofFR-4 and 60 mil thick; it includes four vias, distributed on the boardto match the layout of the version two implantable device boards.

Gold header pins (Pin Strip Header, 3M, Austin, Tex.) were soldered intothe vias so that they extended from the board on both sides of theboard. This enabled us to place our devices onto the test bed, and tapinto the implantable by accessing the header pins. Next, to insulate thevias, plastic caps made out of polyethylene terephthalate (PETG) were 3Dprinted (Flashforge Creator X, FlashForge, Jinhua, China). These capswere printed with a groove so that an O-ring could be placed inside thegroove and create a waterproof seal around the header pins. The capswere connected to the board and compression was created by drilling 2 mmholes through the PCB and cap using a micro-mill (47158, Harbor Freight,Camarillo, Calif.) and screwing the cap and board together. Wiresextending from the testbed were soldered to the header pins and the pinswere then encapsulated. To measure the effectiveness of the seal, theboards were submerged in an aqueous 6 M NaCl solution and the resistancebetween the pins was measured using a Keithley 2400. A MATLAB script waswritten to automatically record and plot the resistance over time. Adrop in the resistance would indicate that the seal was broken. As anadditional test, a piece of litmus paper was also put under the plasticcap with the intention that if the cap leaked, the litmus paper wouldchange color. The pins were encapsulated using the same medical gradeepoxy used to encapsulate the implantable device boards, and parylenewas deposited over the epoxy on the back side of the testboards for acompletely waterproof barrier. The resistance between the twoneighboring pins of the testbed submerged in salt water solution as afunction of time for only epoxy insulation and epoxy plus paryleneinsulation was measured. Without a parylene barrier, the epoxy began toleak, allowing salt water to short out the pins of the testbed.

One version of the implantable device was 1 mm×3 mm×1 mm PCBs made ofFR-4 with a PZT piezoelectric, silicon AS!C, and encapsulated usingcrystal bond. These were implanted into a the sciatic nerve of an Adultmale Long-Evans rat anesthetized with a mixture of ketamine and xylazineIP. A ground truth measurement was obtained using a tungsten microwirewith a 28 G stainless steel needle electrode placed in the foot of theanimal as a reference. Nerve activity was evoked using electricalstimulation and backscatter data was acquired by sending and receivingpressure waves using a transducer (V323-SU-F1 Olympus, Waltham, Mass.).

The original signal across the dust mote was later calculated from thebackscatter data using MATLAB. A representative trace of thereconstructed signal versus the ground truth is shown in FIG. 16.

The reconstructed implantable device data followed the general profileof the ground truth, capturing the compound action potential of thenerve, but several features present in the reconstructed data (such asthe “dips” found from the first to third second) could not be explained.

A second version of the implantable device was roughly 0.8 mm×3 mm×1 mmand used a polyimide substrate and medical-grade UV curable epoxy asencapsulation. A crucial change was the addition of test leads 1 in.long, allowing the voltage across the piezoelectric element to bemeasured, as well as take ground truth measurements by tapping into therecording electrodes. The same device implantation protocol was used inversion two as was used in version one, but reconstruction of thebackscattered signal was done on the fly using a custom transceiverboard.

Example 3—Implantable Devices Encapsulated in Silicon Carbide

Rather than an epoxy encapsulant, silicon carbide (SiC) may be a moreeffective material for insulating and protecting the implantable device.SiC is formed by the covalent bonding of Si and C, forming tetrahedrallyoriented molecules with short bond length and thus, high bond strength,imparting high chemical and mechanical stability. Amorphous SiC (a-SiC)has been welcomed by the biomedical community as a coating material asit can be deposited at much lower temperatures than ordinarily requiredby crystalline SiC and is an electrical insulator. Deposition of a-SiCis generally performed via plasma enhanced chemical vapor deposition(PECVD) or sputtering. Ongoing research using sputtered a-SiC has shownthat it is difficult to achieve a pinhole free layer of SiC. Rather,PECVD using SiH₄ and CH₄ as precursors is capable of yieldingimpressive, pinhole free SiC films.

Furthermore, implanted a-SiC has shown impressive biocompatibility.Previous studies have shown that a 50 μm iridium shaft coated with a-SiCimplanted in the rabbit cortex for ˜20 days did not show the usualchronic inflammatory response of macrophage, lymphocyte, monocyterecruited to the insertion site. See Hess et al., PECVD silicon carbideas a thin film packaging material for microfabricated neural electrodes,Materials Research Society Symposium Proceedings, vol. 1009, doi:10.1557/PROC-1009-U04-03 (2007).

It is interesting to consider an approach to implantable devices thatwould involve constructing the devices on silicon with a silicon carbideencapsulant for a truly chronic implant. A possible process is shown inFIG. 17. One of the largest challenges here is ensuring that the PECVDof SiC dues not depole the piezoelectric material. In order to havecontamination-free films, it is important to deposit at a minimumtemperature of 200° C., but below the Curie temperature of thepiezoelectric transducer.

Example 4—Power Transfer to and Backscatter of a Miniaturized UltrasonicTransducer

A set of experiments were carried out with PZT due to the relative easeof obtaining PZT crystals with varying geometry. Metalized PZT sheets ofseveral thicknesses were obtained (PSI-5A4E, Piezo Systems, Woburn,Mass. and PZT 84, APC Internationals, Mackeyville, Pa.), with a minimumPZT thickness of 127 μm. The PZT was fully encapsulated in PDMS siliconfor biocompatibility.

The most commonly used method to dice PZT ceramics is to use a waferdicing saw with an appropriate ceramic blade to cut PZT sheets intoindividual PZT crystals. The minimum resolution of the cut is determinedby the kerf of the blade and can be as small as 30 μm.

Another possible option is to use a laser cutter. Unlike the dicing saw,laser cutting realizes the cuts by focusing a high-power laser beam ontoa material, which melts, vaporizes, removes, and scribes the piece. Theprecision of laser cutting can be down to 10 μm and is limited by thewavelength of the laser. However, for treating sensitive samples such asPZT ceramics, the temperature at the site of cuts can be damaging to thepiezoelectric performance of the material. Excimer laser cutting ofceramics uses UV laser to cut with excimer from noble gases, but suchlaser cutter is extremely expensive and no suitable services arecurrently available. As a result, a dicing saw was used to perform allthe cuts.

In order to drive or extract electrical energy from the PZT, anelectrical connection is made to both the top and bottom plates. Thematerials typically used as an electrode for PZT are silver or nickel.Silver is generally used for a wide variety of non-magnetic and ACapplications and silver in the form of flakes suspended in a glass fritis usually screened onto the ceramic and fired. For high electric fieldDC applications, silver is likely to migrate and bridge the two plates.As a result, nickel, which has good corrosion resistance and does notelectro-migrate as readily can be electroplated or vacuum deposited asan alternative.

Both materials can be soldered onto with the appropriate solder andflux. For instance, silver is soluble in tin, but a silver loaded soldercan be used to prevent scavenging of silver in the electrode. Phosphorcontent from the nickel plating can make soldering tricky, but thecorrect flux can remove surface oxidation. However, when soldering, inorder to avoid exceeding the Curie point and depoling the PZT sample,the soldering temperature must be between 240 and 300° C. Even at thesetemperatures, since the PZT is also pyroelectric, one must be carefulnot to exceed 2-4 seconds of soldering time.

Alternatively, an electrical connection can be made using either silverepoxy or low temperature soldering using solder paste. Standard two-partsilver epoxy can provide a sufficient electrical conductivity and can becured even at room temperature overnight. However, the joints tend to befragile and can easily break during testing. The bond can be reinforcedby using a non-conductive epoxy as an encapsulation but this additionallayer presents a mechanical load to the PZT and can significantly dampenits quality factor. Low-temperature solder paste on the other handundergoes a phase change between the temperature of 150 and 180° C. andcan provide great electrical connection and a bond strength that iscomparable to that achieved with flash soldering. Therefore, thelow-temperature soldering approach was used.

Wafer dicing is capable of cutting PZTs into small crystals of 10's ofμm. However, samples that are smaller than 1 mm in dimension areextremely difficult to handle with tweezers and bond to. In addition,due to the variation in the length of wire used to interface with topand bottom plates of PZT crystals (and therefore parasitic inductanceand capacitance introduced by the wire) and the amount of solder pastedispensed across a number of samples, the impedance spectroscopemeasurements were inconsistent.

Therefore, a 31 mil thick two-layer FR-4 PCB where all of the electricalinterconnects short and de-embed out the parasitics from the wires andthe board was fabricated. The fabricated board, which includes numeroustest structures and a module for individually characterizing 127 μm, 200μm, and 250 μm thick PZT crystals are shown with dimensions in FIG. 18.Each unit cell in the test module contains two pads with specifieddimensions on one side of the PCB to interface with the PZT crystals andpads for discrete components for backscattering communication on theopposite side. The pitch between the unit cells is limited by the sizeof the discrete components and is roughly 2.3 mm×2 mm.

In order to avoid directly handling tiny PZT crystals, FIGS. 19A-Eoutline a scalable process flow to bond PZT onto the PCB. As shown inFIG. 19A, the solder paste is dispensed using a pump at a constantpressure and for a controlled amount of time on one of the pads on thetop side. The pads are either 250 μm², 200 μm², or 127 μm² based on thethickness of the PZT used. FIG. 19B shows a PZT piece larger than thepad (that can be easily handled) is placed on top to cover the pads. Theboard and piezo assembly is baked in an oven to cure the solder paste.Therefore, PZT crystals are now bonded to pre-soldered bumpedelectrodes. FIG. 19C shows a wafer dicing saw makes a total of four cutsalong the edges of the pad with the solder paste using alignment markerson the board, with non-bonded areas dropping off and leaving an array ofsmall PZT crystals bonded to the PCB. FIG. 19D shows single wirebondmakes an electrical contact between the top plate of the PZT and anelectrode on the PCB, completing the circuit. Finally, FIG. 19E showsthe entire assembly is encapsulated in PDMS (Sylgard 184, Dow Corning,Midland, Mich.) to protect the wirebond and provide insulation.

Since piezoelectric material is an electro-mechanical structure, itselectrical and mechanical properties were characterized. The followingdetails the test setup and techniques to perform such measurements.

Any electrical device can be modeled as a black box using a mathematicalconstruct called two-port network parameters. The properties of thecircuits are specified by a matrix of numbers and the response of thedevice to signals applied to its input can be calculated easily withoutsolving for all the internal voltages and currents in the network. Thereare several different types of two-port network parameters, such asZ-parameters, Y-parameters, S-parameters, and ABCD-parameters, etc. andthe conversion between different parameters can be easily derived. Theapparatus that enables us to extract these parameters is called a vectornetwork analyzer (VNA). A VNA incorporates directional couplers todecompose the voltage in each port into incident and reflected waves(based on impedance mismatching), and calculate the ratio between thesewaves to compute scattering or S-parameters.

Before performing measurements using a VNA, one must calibrate theinstrument since the internal directional couples are non-ideal.Calibration also allows us to move the reference plane of themeasurement to the tips of the cable, i.e., calibrate out parasiticsfrom the cable. There are several calibration standards but the mostcommonly used is open, short, and load calibration procedures. Themeasurement schematic is shown in FIG. 20. Alligator clips, which aresoldered onto the ends of the coaxial cable, are used to interface withthe top/bottom plates. The parasitics from the clips were notsignificant below 100 MHz.

As an example, a VNA (E5071C ENA, Agilent Technologies, Santa Clara,Calif.) was used to measure the electrical properties of a (250 μm)³ PZTcrystal. It was noted that the measured capacitance of the PZT crystalvastly differs from the capacitance expected from a simpleparallel-plate capacitance model due to significant parasiticcapacitances from the PCB and the fixture (clip and connector). Sincethe VNA coefficients from the calibration step previously outlined onlymoved the measurement plane to the tips of the cable, open/short/loadcalibration structures fabricated on the same board were used to includethe board and fixture parasitics. The measured PZT response matched theexpected response after calibration.

Using this calibration technique, the impedance of the PZT can beplotted as a function of frequency, as shown in FIG. 21B. From thisplot, however, it is extremely difficult to determine whether there isany electro-mechanical resonance. When the simulation result with airbacking (no mechanical clamping) was overlaid, it was noticed that theimpedance spectroscopy matches well with the measurement at low and highfrequencies, with the exception of noticeable peak at resonant frequencyof roughly 6 MHz and its harmonics. Upon clamping and loading one sideof PZT with PCB (FR-4), it was seen that a significant dampening of theresonant peaks from air backing. Despite a lack of observable resonancein the measurement, a small blimp around 6 MHz was observed, and themechanical quality factor Q_(m) can be calculated using the followingequations,

$Q_{m} = \frac{f_{a}^{2}}{2Z_{r}{C_{p}\left( {f_{a}^{2} - f_{r}^{2}} \right)}}$

where f_(a) and f_(r) represent anti-resonant (where impedance ismaximized) and resonant frequency (where impedance is minimized), Z_(r)represents an impedance at resonance, and C_(p) is the low-frequencycapacitance. The calculated quality factor from the measurement isroughly 4.2 compared to 5.1 in simulation. According to the datasheet,the unloaded Q of the PZT is ˜500, indicating that FR-4 backing andwire-bonds are causing significant degradation of the quality factor.Despite the drastic reduction in the mechanical Q of the PZT crystals,experiments showed that the backscattered signal level only decreased byroughly ˜19.

In the electrical characterization setup, the VNA has a built-in signalgenerator to provide the input necessary for characterization. In orderto perform acoustic characterization of PZT, acoustic waves weregenerated and launched onto the sample to use as an input. This can beachieved with commercially available broadband ultrasonic transducers.

FIG. 22 shows the composition of a representative transducer, whichconsists of a piezoelectric active element, backing, and wear plate. Thebacking is usually made from a material with high attenuation and highdensity to control the vibration of the transducer by absorbing theenergy radiating from the back face of the active element while the wearplate is used to protect the transducer element from the testingenvironment and to serve as a matching layer.

Ultrasonic power transfer tests were performed using the home-builtsetup shown in FIG. 23. A 5 MHz or 10 MHz single element transducer (6.3mm and 6.3 mm active area, respectively, ˜30 mm focal distance, Olympus,Waltham, Mass.) was mounted on a computer-controlled 2-axis translatingstage (VelMex, Bloomfield, N.Y.). The transducer output was calibratedusing a hybrid capsule hydrophone (HGL-0400, Onda, Sunnyvale, Calif.).Assembly prototypes were placed in a water container such thattransducers could be immersed in the water at a distance ofapproximately 3 cm directly above the prototypes. A programmable pulsegenerator (33522B, Agilent Technologies Santa Clara, Calif.) and radiofrequency amplifier (A150, ENI, Rochester, N.Y.) were used to drivetransducers at specified frequencies with sinusoidal pulse trains of10-cycles and a pulse-repetition frequency (PRF) of 1 kHz. The receivedsignals were amplified with a radio frequency amplifier(BT00500-AlphaS-CW, Tomco, Stepney, Australia), connected to anoscilloscope (TDS3014B, Tektronix, Beaverton Oreg.) to collectultrasound signal and record them using MATLAB.

FIG. 24A and FIG. 24B show a representative measurement of the outputpower of the 5 MHz transducer as a function of the distance between thesurface of the transducer and the hydrophone (z-axis). The peak pressurein water was obtained at ˜33 mm away from the transducer's surface (FIG.24A), while the de-rated peak (with 0.3 dB/cm/MHz) was at ˜29 mm (FIG.24B). FIG. 25A shows the de-rated XZ scan of the transducer output,which show both near-field and far-field beam patterns and a Rayleighdistance or a focal point at ˜29 mm, matching the de-rated peak in FIG.24B. FIG. 25B shows a XY cross-sectional scan of the beam at the focalpoint of ˜29 mm, where the 6 dB beamwidth measured roughly 2.2 mm.

The total integrated acoustic output power of the transducer at variousfrequencies over the 6 dB bandwidth of the beam was nominally kept at aspatial-peak temporal-average I_(SPTA) of 29.2 μW/cm², resulting in atotal output power of ˜1 μW at the focal point, with a peak rarefactionpressure of 25 kPa and a mechanical index (MI) of 0.005. Both thede-rated I_(SPTA) and MI were far below the FDA regulation limit of 720mW/cm² and 1.9, respectively (FDA 2008).

FIG. 21A shows the measured power delivery efficiency of the fullyassembled prototype with cable loss calibrated out for variousimplantable device transducer sizes as compared to analyticalpredictions made for this same setup. Measured results matched thesimulated model behavior very closely across all transducer sizes, withthe exception of a few smaller transducer dimensions, likely due to thesensitivity to transducer position and the ultrasound beamwidth. Themeasured efficiency of the link for the smallest PZT crystal (127 μm)³was 2.064×10⁻⁵, which resulted in 20.64 μW received at the transducernominally. A maximum of 0.51 μW can be recovered at the transducer ifthe transmit output power density was kept at 720 mW/cm². Such low powerlevel harvested by the PZT is mainly due to the extreme inefficiency ofbroadband transducers that were used for the experiments; dedicated,custom-made transducers at each transducer dimension with optimalelectrical input impedance could result in more than 2 orders ofmagnitude improvement in the harvested power level as predicted by thesimulation model.

The frequency response of electrical voltage harvested on a (250 μm)³PZT crystal is shown in FIG. 21C. The resonant frequency was measured tobe at 6.1 MHz, which matches the shift in the resonant frequencypredicted for a cube due to Poisson's ratio and the associated modecoupling between resonant modes along each of the three axes of thecube. Furthermore, the calculated Q of 4 matched the electricallymeasured Q of the PZT.

The experimental result indicate that the analytical model for powercoupling to very small PZT nodes using ultrasound is accurate down to atleast ˜100 μm scale and likely lower. It remains to be seen just howmall a transducer can be fabricated before loss of function. Note thatmeasurements of even smaller nodes (<127 μm) were limited not by theprototype assembly process but by commercial availability of PZTsubstrates. Moving forward, the considerable volume of research andtechniques that has gone into micro- and nanoelectromechanical RFresonators was be used (see Sadek et al., Wiring nanoscale biosensorswith piezoelectric nanomechanical resonators, Nano Lett., vol. 10, pp.1769-1773 (2010); Lin et al., Low phase noise array-compositemicromechanical wine-glass disk oscillator, IEEE Elec. Dev. Meeting, pp.1-4 (2005)) and thin-film piezoelectric transducer (seeTrolier-McKinstry et al., Thin film piezoelectrics for MEMS, J.Electroceram., vol. 12, pp. 7-17 (2004)) to facilitate extremely small(10's of μm) transducers and to truly assess the scaling theory.

Example 5—Beamforming Using Interrogator Ultrasonic Transducer Array

In this example, an ultrasonic beamforming system capable ofinterrogating individual implantable sensors via backscatter in adistributed, ultrasound-based recording platform is presented. A customASIC drives a 7×2 PZT transducer array with 3 cycles of 32V square wavewith a specific programmable time delay to focus the beam at the 800 μmneural dust mote placed 50 mm away. The measured acoustic-to-electricalconversion efficiency of the receive mote in water is 0.12% and theoverall system delivers 26.3% of the power from the 1.8V power supply tothe transducer drive output, consumes 0.75 μJ in each transmit phase,and has a 0.5% change in the backscatter per volt applied to the inputof the backscatter circuit. Further miniaturization of both the transmitarray and the receive mote can pave the way for a wearable, chronicsensing and neuromodulation system.

In this highly distributed and asymmetric system, where the number ofimplanted devices outnumbers the interrogating transceivers by an orderof magnitude, beamforming can be used to efficiently interrogate amultitude of implantable devices. Research into beamforming algorithms,trade-offs, and performance in the implantable device platform hasdemonstrated that cooperation between different interrogators is usefulfor achieving sufficient interference suppression from nearbyimplantable devices. See Bertrand et al., Beamforming approaches foruntethered ultrasonic neural dust motes for cortical recording: asimulation study, IEEE EMBC, 2014, pp. 2625-2628 (August 2014). Thisexample demonstrates a hardware implementation of an ultrasonicbeamforming system for the interrogator and implantable device systemshown in FIG. 2A. The ASIC (see, e.g., Tang et al., Integratedultrasonic system for measuring body-fat composition, 2015 IEEEInternational Solid-State Circuits Conference—(ISSCC) Digest ofTechnical Papers, San Francisco, Calif., 2015, pp. 1-3 (February 2015);Tang et al., Miniaturizing Ultrasonic System for Portable Health Careand Fitness, IEEE Transactions on Biomedical Circuits and Systems, vol.9, no. 6, pp. 767-776 (December 2015)), has 7 identical channels, eachwith 6 bits of delay control with 5 ns resolution for transmitbeam-forming, and integrates high-voltage level shifters and areceive/transmit switch that isolates any electrical feed-through.

The ASIC operates with a single 1.8V supply and generates a 32V squarewave to actuate piezoelectric transducers using integrated charge pumpsand level shifters. The system delivers ˜32.5% of the power from the1.8V supply to the 32V output voltage and ˜81% from 32V to the outputload (each transducer element is 4.6 pF). The ASIC block diagram isshown in FIG. 2A; the circuit details to enable such low energyconsumption per measurement can be found in Tang et al., Integratedultrasonic system for measuring body-fat composition, 2015 IEEEInternational Solid-State Circuits Conference—(ISSCC) Digest ofTechnical Papers, San Francisco, Calif., 2015, pp. 1-3 (February 2015).The ASIC is fabricated in 0.18 μm CMOS with high voltage transistors.The chip area is 2.0 mm² and includes the complete system except for thedigital controller, ADCs, and two off-chip blocking capacitors.

The design of a transducer array is a strong function of the desiredpenetration depth, aperture size, and element size. Quantitatively, theRayleigh distance, R, of the array can be computed as follows:

$R = \frac{D^{2}}{4\lambda}$

where D is the size of the aperture and λ is the wavelength ofultrasound in the propagation medium. By definition, Rayleigh distanceis the distance at which the beam radiated by the array is fully formed;in other words, the pressure field converges to a natural focus at theRayleigh distance and in order to maximize the received power, it ispreferable to place the receiver at one Rayleigh distance where beamspreading is the minimum.

The frequency of operation is optimized to the size of the element. Apreliminary study in a water tank has shown that the maximum energyefficiency is achieved with a (800 μm)³ PZT crystal, which has aresonant frequency of 1.6 MHz post-encapsulation, resulting in λ ˜950μm. The pitch between each element is chosen to be an odd multiple ofhalf wavelength in order to beamform effectively. As a result, for thisdemonstration of beamforming capabilities, the overall aperture is ˜14mm, resulting in the Rayleigh distance of 50 mm. At 50 mm, given theelement size of 800 μm, each element is sufficiently far from the field(R=0.17 mm); therefore, the beam pattern of individual element should beomni-directional enough to allow beamforming.

There are several transmit and receive beamforming techniques that canbe implemented. In this paper, time delay-and-sum transmit beamformingalgorithm is chosen, such that the signals constructively interfere inthe target direction. This algorithm is capable of demonstratingbeam-steering and maximal power transfer to various implantable devices.In order to accommodate backscatter communication to multipleimplantable devices simultaneously, more sophisticated algorithms may berequired. These can include delay-and-sum beamforming, linearlyconstrained minimum-variance beamforming, convex-optimized beamformingfor a single beam, ‘multicast’ beamforming w/convex optimization,maximum kurtosis beamforming, minimum variance distortionless responserobust adaptive beamforming, polyadic tensor decomposition, anddeconvolution of mote impulse response from multi-Rx-channel time-domaindata. The detailed treatment of one aspect of this problem is describedin Bertrand et al., Beamforming approaches for untethered ultrasonicneural dust motes for cortical recording: a simulation study, IEEE EMBC,2014, pp. 2625-2628 (August 2014).

Each of the 7 channels is driven by 3 cycles of 32V square wave with aspecific programmable time delay such that the energy is focused at theobservation distance of 50 mm. The time delay applied to each channel iscalculated based on the difference in the propagation distance to thefocus point from the center of the array and the propagation speed ofthe ultrasound wave in the medium.

Ultrasim was used to characterize the propagation behavior of ultrasoundwave in water with the 1D array described above. Simulated XY (FIG. 26A)and XZ (FIG. 26C) cross-sectional beam patterns closely match themeasurement as shown, despite not modeling the PDMS encapsulation.

Water is used as the medium for measuring the beamforming system as itexhibits similar acoustic properties as the tissue. Pre-metalized LeadZirconate Titanate (PZT) sheets (APC International, Mackeyville, Pa.)are diced with a wafer saw to 800 μm×800 μm×800 μm crystals (parallelcapacitance of 4.6 pF each), which is the size of each transmit element.Each PZT element is electrically connected to the corresponding channelin the ASIC by using a conductive copper foil and epoxy for the bottomterminal and a wirebond for the top terminal. The array is encapsulatedin PDMS (Sylgard 184, Dow Corning, Midland, Mich.) to protect thewirebond and provide insulation. The quality factor of the PZT crystalpost encapsulation is ˜7. The array is organized into 7 groups of 2×1elements, with the pitch of ˜5/2λ˜2.3 mm. The array measuresapproximately 14 mm×3 mm. Finally, the entire assembly is encased in acylindrical tube with the diameter of 25 mm and the height of 60 mm andthe tube is filled with water.

The transducer array's 2D beam pattern and output are calibrated using acapsule hydrophone (HGL-0400, Onda, Sunnyvale, Calif.). The hydrophoneis mounted on a computer-controlled 2D translating stage (VelMex,Bloomfield, N.Y.). The hydrophone has an acceptance angle (−6 dB at 5MHz) of 30°, which is sufficient to capture the beam given thetransmission distance of 50 mm and the scan range (±4 mm).

The measured XY cross-sectional beam pattern with the overlay of thearray is shown in FIG. 26A. The applied delay for each transducer in thearray (element) is shown in FIG. 26B. The −6 dB beamwidth at the focalpoint is 3.2 mm˜3λ. The flexibility of the ASIC allows for both wide andgranular programming of the delays. The peak pressure level of the arrayat 50 mm before and after beamforming is ˜6 kPa and ˜20 kPa,respectively. The 3× in the transmitted output pressure wave afterbeamforming matches the simulation. The simulation also verifies thatthe Rayleigh distance of the array is at 50 mm as shown in FIG. 26C.

Additionally, in order to verify the capability to interrogate multipleimplantable devices, it was verified the beam steering capability of thearray as shown in FIG. 27A (showing beam steering at three differentpositions in the XY-plane), with the time delay for each beam positionshown underneath in FIG. 27B. The 1D beam steering matches very closelywith the simulation, as shown in FIG. 27C. Note that the beam steeringrange is limited to ±4 mm due to the mechanical construct of the array,rather than the electronic capability.

The hydrophone is replaced with an implantable device (with a 800 μm×800μm×800 μm bulk piezoelectric transducer) and placed at the transmissiondistance of 50 mm to verify the power link. The open-circuitpeak-to-peak voltage measured at the mote is 65 mV, for a transmitpulse-duration of 2.56 μs. The spatial peak average acoustic powerintegrated over the ˜6 dB beamwidth at the focal point is 750 μW, whichis 0.005% of the FDA safety limit. The maximum harvestable power at themote is 0.9 μW, resulting in the measured acoustic-to-electricalconversion efficiency of 0.12%. The measured result is in agreement withthe link model (see Seo et al., Model validation of untetheredultrasonic neural dust motes for cortical recording, J. Neurosci.Methods, vol. 244, pp. 114-122 (2015)). The system delivers 26.3% of thepower from the 1.8V power supply to the transducer drive output (definedas driving efficiency) and consumes 0.75 μJ in each transmit phase.

The ultrasonic backscatter communication capability of the system isverified by measuring the difference in the backscattered voltage levelas the input to the backscatter circuit (see Seo et al., Modelvalidation of untethered ultrasonic neural dust motes for corticalrecording, J. Neurosci. Methods, vol. 244, pp. 114-122 (2015)), and isadjusted with a DC power supply. The transmit time and the period of thesystem are 3 μs and 80 μs, leaving a ˜77 μs window for reception. A 2×1element in the center of the array is used for receiving thebackscatter. The output of the receive crystals is amplified anddigitized for processing. The measured backscatter sensitivity is ˜0.5%per volt applied to the input of the backscatter circuit, which is inagreement with the simulation. The overall performance of the system issummarized in Table 5.

TABLE 5 Summary of System Performance Supply voltage 1.8 V Outputvoltage  32 V Number of channels 7 Operating frequency 1.6 MHz Chargepump + level shifter efficiency 26.3% Acoustic-to-Electrical efficiency0.12% Backscatter change 0.5%/V Energy per transmit phase 0.75 μJ

Our measurements with the ultrasonic beamforming system suggest thattransmit beamforming alone can provide sufficient signal-to-noise ratio(SNR) to enable multiple sensors interrogation in the neural dustplatform. The decrease in the SNR with the miniaturization of the dustmote can be largely mitigated by implementing receive beamform.Furthermore, in order to increase the rate of interrogation, one couldexplore an alternative means of multiplexing, such as spatialmultiplexing where multiple motes are interrogated simultaneously withthe same transmit beam. However, it is important to consider the systemdesign tradeoff between processing/communication burden to powerconsumption. Additionally, sufficient suppression of interferences fromnearby dust motes is necessary to achieve the required SNR.

The acoustic-to-electrical efficiency at 0.12% currently dominates theefficiency

$\left( \frac{P_{harvested}}{P_{1.8V\mspace{14mu} {supply}}} \right)$

of the overall system. Despite such low efficiency of the power link, if˜1% of the FDA safety regulation (spatial peak average of 1.9 W/cm²) canbe outputted, it is possible harvest up to 0.92V peak-to-peak voltageand 180 μW at the 800 μm ultrasonic transducer 50 mm away in water.

Furthermore, the low efficiency of the power link in this demonstrationis attributed to such large transmission distance, as determined by thearray aperture and the element size. For peripheral nerve intervention,for example, the desired transmission distance is approximately 5 mm,which includes the thickness of skin, tissue, etc. In order to be at thefar field of the array, the aperture should be ˜4.4 mm. Further scalingof each element can reduce the overall dimensions of the array apertureand the transmission distance down to the desired 5 mm. Simulationindicates that acoustic-to-electrical efficiency up to 1% can beachieved in water with a 100 μm receive ultrasonic transducer.

For transmission in tissue, assuming 3 dB/cm/MHz loss in tissue, FIG. 28shows the scaling of both link efficiency and received power level givenoperation at 1% of the FDA safety limit. Despite this ratherconservative loss, at 100 μm, the simulation indicates that it ispossible to harvest up to 0.6V peak-to-peak voltage and 75 μW.Therefore, wireless power transfer in tissue using this platform isfeasible. Furthermore, this power level is sufficient to operate highlyefficient, low-power energy harvesting circuits and charge pumps,similar to the ASIC presented here, to output voltages that are suitablefor electrically stimulating nearby neurons and detecting physiologicalconditions using sensors.

Example 6—Wireless Recording in the Peripheral Nervous System withUltrasonic Neural Dust

The following example demonstrates implantable device systems forrecording neural signals. The example shows that ultrasound is effectiveat delivering power to mm scale devices in tissue; likewise, passive,battery-less communication using backscatter enables high-fidelitytransmission of electromyogram (EMG) and electroneurogram (ENG) signalsfrom anesthetized rats. These results highlight the potential for anultrasound-based neural interface system for advancing futurebioelectronics-based therapies. The example further provides methods fordetermining the location and movement of the implantable device.

The implantable device system was used in vivo to reportelectroneurogram (ENG) recordings from the sciatic nerve in a peripheralnervous system, and an electromyogram (EMG) recording from agastrocnemius muscle of a subject rat. The system included an externalultrasonic transceiver board which powers and communicates with mamillimeter-scale sensor implanted not either a nerve or muscle. See FIG.29A. The implantable device included a piezoelectric crystal, a singlecustom transistor, and a pair of recording electrodes. See FIGS. 29B,29C, and 29D.

During operation, the external transducer alternates between a) emittinga series of six 540 ns pulses every 100 μs and b) listening for anyreflected pulses. The entire sequence of transmit, receive andreconstruction events are detailed in FIG. 30A-H; this sequence isrepeated every 100 μs during operation. Briefly, pulses of ultrasonicenergy emitted by the external transducer impinge on the piezocrystaland are, in part, reflected back towards the external transducer. Inaddition, some of the ultrasonic energy causes the piezocrystal tovibrate; as this occurs, the piezocrystal converts the mechanical powerof the ultrasound wave into electrical power, which is supplied to thetransistor. Any extracellular voltage change across the two recordingelectrodes modulates the transistor's gate, changing the amount ofcurrent flowing between the terminals of the crystal. These changes incurrent, in turn, alter the vibration of the crystal and the intensityof the reflected ultrasonic energy. Thus, the shape of the reflectedultrasonic pulses encodes the electrophysiological voltage signal seenby the implanted electrodes and this electrophysiological signal can bereconstructed externally. The performance specifications of neural dustin comparison to other state-of-the-art systems are summarized in Table6.

TABLE 6 This Ref. 1 Ref. 2 Ref. 3 Ref 4 Ref. 5 Example Power SourceWireless Wireless Wireless Wireless Wireless Wireless (RF) (RF) (RF)(RF) (US) (US) Gain 46 dB 30 d — — — N/A Bandwidth 10 kHz 0.5 kHz 3 kHz5 kHz — >30 kHz TX Frequency 1.5 GHz 300 MHz 2.2-2.45 GHz 2.4 MHz 1 MHz1.85 MHz Resolution 10 bits 15 bits — — — 8 bits (digitizer) Noise Floor6.5 μV_(rms) 1.2 μV_(rms) 500 μV_(rms) 63 μV_(rms) — 180 μV_(rms)* #Channels 4 64 1 1 1 1 Total TX Power 50 mW 12 mW 47 mW 40 mW 0.36 mW0.12 mW Avg. Power 2.63 μW 3.52 μW 0 μW 0 μW 85 μW 0 μW (per ch)Wireless Data 1 Mbps 1 Mbps — — — 0.5 Mbps Rate Range in Tissue 0.6 mm10 mm 15 mm 13 mm 30 mm 8.8 mm Volume — — 24 mm³ 360 mm³ 45 mm³ 2.4 mm³(per ch) *In a stationary, water tank setup

An implantable device was manufactured with on a 50 μm thick polyimideflexible printed circuit board (PCB) with an ultrasonic transducerpiezocrystal (0.75 mm×0.75 mm×0.75 mm) and a custom transistor (0.5mm×0.45 mm) attached to the topside of the board with a conductivesilver paste. Electrical connections between the components are madeusing aluminum wirebonds and conductive gold traces. Exposed goldrecording pads on the bottom of the board (0.2 mm×0.2 mm) are separatedby 1.8 mm and make contact on the nerve or muscle to recordelectrophysiological signals. Recorded signals are sent to thetransistor's input through micro-vias. Additionally, some implants wereequipped with 0.35 mm-wide, 25 mm-long, flexible, compliant leads withtest points for simultaneous measurement of both the voltage across thepiezocrystal and direct wired measurement of the extracellular potentialacross the electrode pair used by the ultrasonic transducer (thisdirect, wired recording of extracellular potential as the ground truthmeasurement is referred to below, which is used as a control for theultrasonically reconstructed data). The entire implant is encapsulatedin a medical grade UV-curable epoxy to protect wirebonds and provideinsulation. A single implantable device measures roughly 0.8 mm×3 mm×1mm. The size of the implants is limited only by our use of commercialpolyimide backplane technology, which is commercially accessible toanyone; relying on more aggressive assembly techniques with in-housepolymer patterning would produce implants not much larger than thepiezocrystal dimensions (yielding a ˜1 mm³ implant).

Further details on implantable device assembly. Lead zirconate titanate(PZT) sheets (841, APC Int., Mackeyvile, Pa.) with ˜12 μm of fired onsilver were diced to desired dimensions using a dicing saw (DAD3240,Disco, Santa Clara, Calif.) with a ceramic blade (PN CX-010-270-080-H).The diced PZT coupon, along with the custom transistor, were attached toa 50 μm thick polyimide flexible PCB with immersion gold (Altaflex,Santa Clara, Calif.) using a thin layer of two-part silver epoxy with1:1 mix ratio (H20E, Epotek, Billerica, Mass.). The board was cured at150° C., which is far below the melting temperature of polyimide and theCurie temperature of the PZT, for 10 minutes. The custom transistor waswirebonded using an aluminum ultrasonic wirebonder (7400B, West Bond,Scotts Valley, Calif.) to pre-patterned targets. In order to preventcharge build-up on the PZT from the wedge contact, top and bottomcontacts of the PZT were discharged to a thin metal sheet prior towirebonding the top contact of the PZT to close the circuits.Medical-grade, UV-curable epoxy (OG116-31, Epotek) was used to protectthe wirebond and provide insulation. The platform was then cured in UVchamber (Flash, Asiga, Anaheim Hills, Calif.) with 92 mW/cm² @ 365 nmfor 3 minutes.

A custom integrated circuit operates the external transceiver board andenables low-noise interrogation. An external, ultrasonic transceiverboard interfaces with the implantable device by both supplying power(transmit (TX) mode) and receiving reflected signals (receive (RX)mode). This system is a low-power, programmable, and portabletransceiver board that drives a commercially available externalultrasonic transducer (V323-SU, Olympus, Waltham, Mass.). Thetransceiver board exhibited a de-rated pressure focus at ˜8.9 mm (FIG.31A). The XY cross-sectional beam-pattern clearly demonstrated thetransition from the near-field to far-field propagation of the beam,with the narrowest beam at the Rayleigh distance (FIG. 31B). Thetransducer was driven with a 5 V peak-to-peak voltage signal at 1.85MHz. The measured de-rated peak rarefaction pressure was 14 kPa,resulting in a mechanical index (MI) of 0.01. De-rated spatial pulsepeak average (I_(SPPA)) and spatial peak time average (I_(SPTA)) of 6.37mW/cm² and 0.21 mW/cm² at 10 kHz pulse repetition were 0.0034% and 0.03%of the FDA regulatory limit, respectively. The transceiver board wascapable of outputting up to 32 V peak-to-peak and the output pressureincreased linearly with the input voltage (FIG. 31C).

Reflections from non-piezocrystal interfaces provide a builtin-reference for movement artifacts and temperature drift. The entiresystem was submerged and characterized in a custom-built water tank withmanual 6 degrees-of-freedom (DOF) linear translational and rotationalstages (Thorlabs Inc., Newton, N.J.). Distilled water was used as apropagation medium, which exhibits similar acoustic impedance as tissue,at 1.5 MRayls. For initial calibration of the system, a current source(2400-LV, Keithley, Cleveland, Ohio) was used to mimic extracellularsignals by forcing electrical current at varying current densitiesthrough 0.127 mm thick platinum wires (773000, A-M Systems, Sequim,Wash.) immersed in the tank. The implantable device was submerged in thecurrent path between the electrodes. As current was applied between thewires, a potential difference arose across the implant electrodes. Thispotential difference was used to mimic extracellularelectrophysiological signals during tank testing.

Further details on electrical and ultrasonic characterization of theassembly in water. The custom transistor was electrically tested with aprecision current meter (2400-LV, Keithley) and a DC-power supply(3631A, Agilent, Santa Clara, Calif.). To characterize the piezocrystalprior to assembly, an impedance plot was obtained with an impedanceanalyzer (4285A, Agilent) using two-terminal measurements withopen/short/load calibration scheme. The impedance of exposed goldrecording pads (0.2 mm×0.2 mm), separated by 1.8 mm on the bottom of thePCB, was measured in Phosphate Buffered Solution (PBS 1×) with anelectrochemical impedance spectroscope (nanoZ, White Matter LLC, MercerIsland, Wash.). The device formed the active electrode and a silver wireformed the reference electrode. Ultrasonic characterization of thetransducer was performed in a custom-built water tank. A capsulehydrophone (HGL-0400, Onda Corp., Sunnyvale, Calif.) with 20 dBpreamplification (AH-2020, Onda Corp.) was mounted on acomputer-controlled 2D translating stage (XSlide, VelMex Inc.,Bloomfield, N.Y.) and was used to calibrate the output pressure andcharacterize beam patterns of a 2.25 MHz single element transducer(V323-SU, Olympus). Verification of ultrasonic power transfer andcommunication sensitivity was performed in a smaller water-tank with thetransducer mounted on manual translational and rotational stages(Thorlabs Inc.). The outline of the implantable device was patterned onan extruded acrylic piece with UV-laser and the implantable device wasclamped to the acrylic stage with nylon screws. The position and angleof the transducer with relative to the mote were manually adjusted untilthe maximum voltage was measured across the piezocrystal. Cablecapacitances and parasitics were carefully calibrated by adjusting theseries capacitance in the high-impedance probes (N2863B, Agilent). Anelectric field in the water tank was generated with a current source(2400-LV, Keithley) forcing electrical current at varying currentdensities through two 0.127 mm thick platinum wires (773000, A-Msystems) immersed in the tank. The transceiver board consisted of acustom integrated circuit (IC) in a QFN-64 package that achieved anon-chip 1.8V to 32V charge pump efficiency of 33% and system latency of20 ns and consumed 16.5 μJ per each transmit cycle (Tang et al., 2015).During the receive mode, the high voltage switch was closed and thesignal was amplified by 28 dB; both operations were performed on-chip.The output signal from the chip was digitized by an off-chip 10-bit, 100MHz analog-to-digital converter (ADC) (LTC2261-12, Linear Technology,Milpitas, Calif.). The outputs of the ADC were fed back into thefield-programmable gate array (FPGA) and USB 3.0 integration module(XEM6310-LX45, Opal Kelly, Portland, Oreg.) and transferred to thelaptop. The FPGA-USB module was also used to serially program the IC.

To interrogate the implantable device, six 540 ns pulses every 100 μswere emitted by the external transducer. See FIG. 30. These emittedpulses reflect off the neural dust mote and produce backscatter pulsesback towards the external transducer. Reflected backscatter pulses wererecorded by the same transceiver board. The received backscatterwaveform exhibits four regions of interest; these are pulses reflectingfrom four distinct interfaces (FIG. 31D): 1) the water-polymerencapsulation boundary, 2) the top surface of the piezoelectric crystal,3) the piezo-PCB boundary, and 4) the back of the PCB. As expected, thebackscatter amplitude of the signals reflected from the piezoelectriccrystal (second region) changed as a function of changes in potential atthe recording electrodes. Reflected pulses from other interfaces did notrespond to changes in potential at the recording electrodes.Importantly, pulses from the other non-responsive regions were used as asignal level reference, making the system robust to motion orheat-induced artifacts (since pulses reflected from all interfaceschange with physical or thermal disturbances of the neural dust mote butonly pulses from the second region change as a function ofelectrophysiological signals). In a water tank, the system showed alinear response to changes in recording electrode potential and a noisefloor of ˜0.18 mVrms (FIG. 31E). The overall dynamic range of the systemis limited by the input range of the transistor and is greater than >500mV (i.e., there is only an incremental change in the current once thetransistor is fully on (input exceeds its threshold voltage) or fullyoff). The noise floor increased with the measured power drop-off of thebeam; 0.7 mm of misalignment degraded it by a factor of two (N=5devices, FIG. 31F). This lateral mis-alignment-induced increase in thenoise floor constitutes the most significant challenge to neuralrecordings without a beamsteering system (that is, without the use of anexternal transducer array that can keep the ultrasonic beam focused onthe implanted dust mote and, thus, on-axis). On axis, the implantabledevice converted incident acoustic power to electrical power across theload resistance of the piezo with ˜25% efficiency. FIG. 31G plots theoff-axis drop-off of voltage and power at one Rayleigh distance for thetransducer used in this example. Likewise, FIG. 31H plots the change ineffective noise floor as a function of angular misalignment.

EMG and ENG can be recorded tetherlessly in-vivo in rodents. EMGresponses from the gastrocnemius muscle of adult Long-Evans rats underanesthesia were recorded using the implantable device system. Theimplantable device (“dust”) was placed on the exposed muscle surface,the skin and surrounding connective tissue were then replaced, and thewound was closed with surgical suture (FIG. 32A). The ultrasonictransducer was positioned 8.9 mm away from the implant (one Rayleighdistance of the external transducer) and commercial ultrasound gel(Aquasonic 100, Parker Labs, Fairfield, N.J.) was used to enhancecoupling. The system was aligned using a manual manipulator bymaximizing the harvested voltage on the piezocrystal measured from theflexible leads. Ag/AgCl wire hook electrodes were placed approximately 2cm distally on the trunk of the sciatic nerve for the bulk stimulationof muscle fiber responses. Stimulation pulses of 200 μs duration wereapplied every 6 seconds and data was recorded for 20 ms around thestimulation window (FIG. 32B). The power spectral density (PSD) of thereconstructed data with several harmonics due to edges in the waveformis shown in FIG. 32C. This process could be continued indefinitely,within the limit of the anesthesia protocol; a comparison of data takenafter 30 minutes of continuous recording showed no appreciabledegradation in recording quality (FIG. 32D).

EMG recruitment curves were obtained with both ground truth and wirelessdust backscatter by varying stimulation amplitude (FIGS. 33A and 33B).Reconstruction of the EMG signal from the wireless backscatter data wassampled at 10 kHz, while the wired, ground truth measurement was sampledat 100 kHz with a noise floor of 0.02 mV. The two signals atresponse-saturating stimulation amplitude (100%) matched with R=0.795(FIG. 33C). The difference between the wireless and wired data waswithin ±0.4 mV (FIG. 33D). The salient feature of the implantable deviceEMG response was approximately 1 ms narrower than the ground truth,which caused the largest error in the difference plot (FIGS. 33C and33D). The responses from skeletal muscle fibers occurred 5 mspost-stimulation and persisted for 5 ms. The peak-to-peak voltage of theEMG shows a sigmoidal response as a function of stimulation intensity(FIG. 33E). The error bars indicate the measurement uncertainties fromtwo rats and 10 samples each per stimulation amplitude. The minimumsignal detected by the implantable device is approximately 0.25 mV,which is in good agreement with the noise floor measurement made in awater tank.

A similar setup was prepared to measure the electroneurogram (ENG)response from the main branch of the sciatic nerve in anesthetized rats.The sciatic nerve was exposed by separating the hamstring muscles andthe neural dust mote was placed and sutured to the nerve, with therecording electrodes making contact with the epineurium. A similargraded response was measured on both ground truth and wirelessbackscatter from the implantable device by varying stimulation currentamplitude delivered to bipolar stainless steel electrodes placed in thefoot (FIGS. 34A and 34B). The two signals at response-saturatingstimulation amplitude (100%) matched with R=0.886 (FIG. 34C); theaverage error was within ±0.2 mV (FIG. 34D). The peak-to-peak ENGvoltage showed a sigmoidal response with the error bars indicatinguncertainties from two rats and 10 samples each per stimulationamplitude. The minimum signal detected by the implantable device wasagain at 0.25 mV (FIG. 34E).

Further details on experiment setup and surgical procedures. All animalprocedures were performed in accordance with University of CaliforniaBerkeley Animal Care and Use Committee regulations. Adult maleLong-Evans rats were used for all experiments. Prior to the start ofsurgery, animals were anesthetized with a mixture of ketamine (50 mg/kg)and xylazine (5 mg/kg) IP. The fur surrounding the surgical site wasshaved and cleaned. For EMG recordings, a patch of gastrocnemius muscleroughly 10 mm×5 mm in size was exposed by removing the overlying skinand fascia. The implantable device was then placed on the exposedmuscle, and the skin and fascia were replaced and the wound was closedwith 5/0 surgical suture. For ENG recordings, the sciatic nerve wasexposed by making an incision from the sciatic notch to the knee, andseparating the hamstring muscles. The implantable device was then placedin contact with the epineurium of the main branch of the sciatic nervebundle, and sutured to the nerve using 10/0 microsurgical suture.Animals were euthanized at the conclusion of the experiments.Constant-current stimulation was delivered using an isolated pulsestimulator (2100, A-M Systems). Single biphasic pulses with a 2 ms pulsewidth were used to deliver stimulation at various current amplitudes.For each experiment, electrophysiological responses from 10 stimulations(i.e., samples) were recorded. The FPGA-USB module generated a triggerfor the stimulator every 6 seconds. For EMG experiments, bipolar Ag—AgClhook electrodes placed around the trunk of the sciatic nerve were usedfor stimulation. To evoke ENG activity, 28 G stainless steel needleelectrodes were placed in the foot with an inter-electrode distance ofapproximately 5 mm. The wired signals were amplified (100×) by abattery-powered differential amplifier with a built-in bandpass filter(DAM50, WPI, Sarasota, Fla.) set at 10 Hz-1 kHz. The ground referencefor the amplifier was a 28 G stainless steel needle electrode placed inthe contralateral foot relative to the recording setup. The output ofthe amplifier was connected to a multi-channel digitizer, sampled at 100kHz, and recorded on computer. The implantable device was placed oneRayleigh distance from the transducer (8.9 mm), which corresponded to5.9 μs transit time, assuming an acoustic velocity of 1500 m/s in water.6-cycles of square waves at 1.85 MHz with peak voltage of 5 V werelaunched every 100 μs (pulse repetition frequency (PRF) of 10 kHz). Thetotal transmit pulse width was approximately, 3.3 μs, which wassufficiently small to prevent any overlaps with the first harvestedvoltage measurement at 5.9 μs. Given that the first reflection back tothe transducer (e.g., backscatter) occurred at approximately 11.8 μs(twice the transit time) and persisted until for 3.3 μs, the maximum PRF(e.g., in this context, the sampling rate) was ˜66 kHz. Given that thebulk peripheral nerve responses occurred below 1 kHz, a PRF of 10 kHzwas chosen to sufficiently capture the dynamics. In order to sample thebackscatter waveform at 1.85 MHz without losing signal fidelity, theoff-chip ADC on the transceiver board was heavily oversampled at 50 MHz.This resulted in ˜8 Mbits of data in a 10 ms neural recording, which wasstored in a 128 MByte, 16-bit wide, synchronous DDR2 DRAM(MT47H64M16HR-3, Micron Technology, Boise, Id.). The raw waveforms weretransferred to the laptop via the USB interface post-recording. The rawwaveforms were simultaneously recorded using an 8-bit digitizer(USB-5133, National Instruments, Santa Clara, Calif.) for comparison.Raw backscatter waveforms, sampled at 50 MHz, from each experiment weresliced and time-aligned to be averaged over samples. The averagedsignals were bandpass-filtered with a symmetric 4th order Butterworthfilter from 10 Hz to 1 kHz. The distinct characteristics of thebackscatter waveform (FIG. 31D) were used as a template to locate theregion of interest. The signals were then rectified and the integral ofthe region was computed to estimate the input voltage signal, whichexhibited a linear response (FIG. 31E). Multiplication factor for thesignal was extracted from the ground truth measurement.

In-vivo ultrasonic transmission. A 2.25 MHz single element transducer(V323-SU, Olympus NDT, Waltham, Mass.) was used to generate 6 pulses at1.85 MHz. The transducer had a measured half-power bandwidth (HPBW) ofmore than 2.5 MHz. In order to measure the transmission loss through thetissue, various thicknesses of skin found near the gastrocnemius muscleof a male Long-Evans rat was placed in between the transducer and theimplantable device. The harvested voltage on the piezocrystal with andwithout tissue was obtained and the 8.9 mm of tissue resulted in 10 dBof tissue attenuation.

ENG recording with different electrode spacing. Recording electrodeswith various spacing were fabricated on a 50 μm thick polyimide flexibleprinted circuit board (PCB). There were a total of 5 electrodes, eachmeasuring 0.2 mm×0.2 mm, and one of them was used as the referenceelectrode. Other electrodes were spaced 0.3 mm, 0.8 mm, 1.3 mm, and 1.8mm, respectively, apart from the reference electrode. The spacing boardwas placed in contact with the epineurium of the main branch of thesciatic nerve bundle (distal) and sutured to the nerve. Bipolar Ag—AgClhook electrodes placed around the trunk of the sciatic nerve (proximal)were used for stimulation. Constant-current simulation of a singlebiphasic pulse with a duration of 0.5 ms every 1 second was deliveredusing an isolated pulse stimulator (2100, A-M Systems, Sequim, Wash.).The recorded signals with various spacing between the electrodes wereamplified (100×) by a battery-powered differential amplifier with abuilt-in bandpass filter (DAM50, WPI, Sarasota, Fla.) set at 10 Hz-1 kHz(FIG. 35A). As expected, the peak-to-peak voltage recorded on theelectrode increased with the spacing at least quadratically. Theamplitude saturated after the spacing of 1.3 mm, confirming that theelectrode spacing of 1.8 mm on the recording sensor was sufficient tocapture the maximum, saturated ENG response (FIG. 35B).

Calculation of acoustic intensity. Several parameters are established bythe American Institute for Ultrasound in Medicine and NationalElectronics Manufacturers Administration (NEMA) to assess the safety ofan ultrasonic system. The acoustic power output of diagnostic ultrasonicsystem is limited by the de-rated values of spatial-peak pulse-averageintensity (ISPPA), spatial-peak temporal average intensity (ISPTA), andmechanical index (MI). These de-rated values are computed by multiplyingthe measured values in water by an attenuation factor of 0.3 dB/cm/MHzto simulate the effects on tissue. A capsule hydrophone (HGL-0400, OndaCorp) with 20 dB preamplification (AH-2020, Onda Corp., Sunnyvale,Calif.) was mounted on a computer-controlled 2D translating stage(XSlide, VelMex Inc., Newton, N.J.) and immersed in a custom-built watertank to calibrate the output pressure of a 2.25 MHz single elementtransducer (V323-SU, Olympus NDT). 6-cycles of square waves at 1.85 MHzwith peak input voltage of 5 V were launched every 1 ms (pulserepetition frequency (PRF) of 10 kHz) to the transducer. The hydrophonewas placed one Rayleigh distance from the transducer (8.9 mm). The pulseintensity integral (PII) is defined as:

${PII} = {\int\frac{p^{2}(t)}{z_{o}}}$

dt where p is the instantaneous peak pressure, and z_(o) is thecharacteristic acoustic impedance of the medium. In the case of water,z_(o) is estimated to be 1.5 MRayl. The I_(SPPA) is defined as:

${I_{SPPA} = \frac{PII}{PD}},$

where PD is the pulse duration defined as (t)(0.9×PII−0.1×PII)×1.25, asoutlined by the standards established by NEMA. The I_(SPPA) is definedas: I_(SPPA)=PII×PRF, where PRF is the pulse repetition frequency. TheMI is defined as:

${{MI} = \frac{p_{r}}{\sqrt{f}}},$

where p_(r) is the peak rarefaction pressure and f is the acousticfrequency.

or in-vivo, acute recordings in a stationary, anaesthetized rat modelwere used to collect compound action potentials from the main branch ofthe sciatic nerve as well as evoked EMG from the gastrocnemius muscle.The performance of the system was equivalent to conventionalelectrophysiological recordings employing microelectrodes and cabledelectronics. One of the principal strengths of the demonstratedtechnology is that, unlike conventional radio frequency technology,ultrasound-based systems appear scalable down to <100 μm size, openingthe door to a new technological path in implantable electronics. Physicslimits how small a good radio frequency receiver can be due to the longwavelengths of radio frequency energy (millimeters to centimeters) andthe high degree of absorption of radio frequency energy into tissue(which heats up the tissue and limits the total power than can be sentto an implant). Ultrasonic systems fare much better in both areas,allowing for the design of extremely small receiver devices. Inaddition, the extreme miniaturization of lower power electronics allowsfor useful recording electronics to be incorporated into such smallpackages. Flat, low-profile piezo-transducer with proper impedancematching would enable a wearable transceiver board small enough forawake, behaving rodent neurophysiology. Additionally, wearable,battery-powered multi-element arrays would allow for beam-steering ofthe ultrasonic beam, with several advantages: 1) the implantable devicescould be maintained on-axis even in the face of relative motion betweenthe implantable device and external transducer; 2) multiple implantabledevices could be interrogated by sweeping the focused beamelectronically; and 3) post-surgical tuning of the implantable devicelocation would be made easier. Additional de-noising of the transceiverdrive electronics should also help decrease the noise floor. Inaddition, the calculated scaling predictions suggest that <500 μm scaleimplantable devices are feasible. To do this, a number of material andmicrofabrication challenges exist, including the use of microfabricatedbackplanes, solder microbumping assembly of components (instead of theconventional wirebonding approach used here) and the use of thin filmencapsulants (instead of medical grade epoxy) such as parylene.Transitioning away from PZT piezocrystals to biocompatible BaTiO3 singlecrystal transducers is also contemplated; taken together, thesedevelopments would open the way for chronic studies of neural andmuscular tissue recording.

Example 7—Digital Communication Link Between Implantable Device andInterrogator

A system including an implantable device and an interrogator having atransducer array is validated with a bench-top setup mimicking anin-vivo environment. Ultrasound coupling gel serves as a tissue phantomdue to its acoustic impedance which is similar to that of targetbiological tissues (approximately 1.5 MRayl). An implantable device witha bulk piezoelectric transducer with direct connections to the twoelectrodes contacting the transducer is placed in the tissue phantom,and the interrogator transducer array is coupled to the gel. Bothelements are attached to precision controlled stages for accuratepositioning. The transducer array is placed 14 mm away from the dustmote, which corresponds to a 18.6 μs round-trip time of flight assumingan acoustic velocity of 1,540 m/s in ultrasound coupling gel. Thetransducer array is excited with six 1.8 MHz, 0-32 V rectangular pulses,and the backscatter signal is digitized with 2000 samples at 17 Msps and12-bits of resolution. For time-domain backscatter inspection, completebackscatter waveforms are filtered in real time on the device and sentto the client through a wired, serial connection. In normal operation,the complete modulation extraction algorithm is applied to thebackscatter data on the device in real-time, compressing the backscattersignal to four bytes. The processed data is transmitted throughBluetooth's SSP protocol to a remote client and streamed through the GUIin real-time.

FIG. 36A shows the filtered backscatter signals collected with thedescribed experimental setup. Signals are collected while the dust motepiezocrystal electrodes are in the shorted and opened configurations.The change in impedance due to the switch activity results in abackscatter peak amplitude that is 11.5 mV greater in the open switchconfiguration, a modulation depth of 6.45%. (FIG. 36B). The longduration of the echo from the mote indicates transducer ringing despitea damping backing layer. While the under-damped transducer systemresponse does spread out the backscatter signal in the time-domain,demodulation is successful as long as the backscatter from the implanteddevice is captured within the ROI.

Using pulse-amplitude-modulated non-return to zero level coding, abackscatter sensor mote is modulated to send a predetermined11-character ASCII message (“hello world”). The modulation of thedevice's acoustic impedance is achieved by shunting the piezoelectrictransducer across a digitally controlled switch where a high levelcorresponds to the open configuration and a low level corresponds to theclosed configuration. FIG. 37 shows the modulated values on thetransducer and the corresponding extracted modulation values of theinterrogator. The absolute value and noise margin of the extractedsignal values depend on a variety of factors such as mote distance,orientation, and size; however, the extracted waveform remainsrepresentative of the modulated signal on the dust mote, varying by alinear scaling factor.

Wirelessly transmitting the extracted backscatter value of theimplantable device modulated by “hello world” demonstrates the device'sreal time communication link with implanted devices. Interrogation of atwo state backscatter system provides a robust demonstration of thesystem's wireless communication link with both an implantable sensor anda remote client. This wireless communication link invites developmentstoward closed-loop neuromodulation systems to connect the brain withexternal devices.

1. A closed-loop system, comprising: (a) a first device configured todetect a signal; (b) an interrogator comprising one or more ultrasonictransducers configured to receive the ultrasonic backscatter encodingthe signal and emit ultrasonic waves encoding a trigger signal; and (c)a second device configured to emit an electrical pulse in response tothe trigger signal, wherein the second device is implantable,comprising: an ultrasonic transducer configured to receive ultrasonicwaves, wherein the received ultrasonic waves power the second device andencode a trigger signal; a first electrode and a second electrodeconfigured to be in electrical communication with a tissue and emit anelectrical pulse to the tissue in response to the trigger signal; and anintegrated circuit comprising an energy storage circuit.
 2. The systemof claim 1, wherein the signal is a physiological signal.
 3. The systemof claim 1, wherein the signal comprises an electrophysiological signal.4. The system of claim 1, wherein the signal comprises a temperature, apH, a pressure, a strain, or a bioimpedance.
 5. The system of claim 1,wherein the first device is implantable.
 6. The system of claim 1,wherein the integrated circuit of the second device comprises a digitalcircuit.
 7. The system of claim 1, wherein the integrated circuit of thesecond device comprises a mixed-signal integrated circuit configured tooperate the first electrode and the second electrode.
 8. The system ofclaim 1, wherein the integrated circuit comprises a power circuitcomprising the energy storage circuit.
 9. The system of claim 1, whereinthe tissue comprises muscle tissue or an organ.
 10. The system of claim1, wherein the tissue comprises nervous tissue.
 11. The system of claim1, wherein the first device and the second device are implanted in asubject.
 12. The system of claim 11, wherein the subject is a human. 13.The system of claim 11, wherein the interrogator is wearable by thesubject.
 14. A closed-loop system, comprising: (a) a first implantabledevice configured to detect a signal comprising: a sensor configured todetect the signal in a first tissue; an integrated circuit comprising amodulation circuit configured to modulate a current based on thedetected signal, and a first ultrasonic transducer configured to emit anultrasonic backscatter encoding information related to the detectedphysiological signal based on the modulated current; (b) an interrogatorcomprising one or more ultrasonic transducers configured to receive theultrasonic backscatter encoding the information related to the detectedphysiological signal, and emit ultrasonic waves encoding a triggersignal; and (c) a second implantable device configured to emit anelectrical pulse to a second tissue in response to the trigger signal,comprising: an ultrasonic transducer configured to receive ultrasonicwaves, wherein the received ultrasonic waves power the second device andencode the trigger signal; a first electrode and a second electrodeconfigured to be in electrical communication with the second tissue andemit an electrical pulse to the second tissue in response to the triggersignal; and an integrated circuit comprising an energy storage circuit.15. The system of claim 14, wherein the first tissue and the secondtissue are the same tissue.
 16. The system of claim 14, wherein thefirst tissue and the second tissue are different tissues.
 17. The systemof claim 14, wherein the sensor comprises a first electrode and a secondelectrode configured to be in electrical communication with the firsttissue.
 18. The system of claim 14, wherein the signal is aphysiological signal.
 19. The system of claim 14, wherein the signalcomprises an electrophysiological signal.
 20. The system of claim 14,wherein the signal comprises a temperature, a pH, a pressure, a strain,or a bioimpedance.
 21. The system of claim 14, wherein the integratedcircuit of the second implantable device comprises a digital circuit.22. The system of claim 14, wherein the integrated circuit of the secondimplantable device comprises a mixed-signal integrated circuitconfigured to operate the first electrode and the second electrode. 23.The system of claim 14, wherein the tissue comprises muscle tissue or anorgan.
 24. The system of claim 14, wherein the tissue comprises nervoustissue.
 25. The system of claim 14, wherein the first device and thesecond device are implanted in a subject.
 26. The system of claim 25,wherein the subject is a human.
 27. The system of claim 25, wherein theinterrogator is wearable by the subject.
 28. A method of stimulating atissue, comprising: receiving ultrasonic waves at a first implantabledevice configured to detect a physiological signal; converting, at thefirst implantable device, energy from the ultrasonic waves into anelectrical current that flows through a modulation circuit; detecting,at the first implantable device, the physiological signal; producing, atthe first implantable device, a modulated electrical current based onthe detected physiological signal; transducing, at the first implantabledevice, the modulated electrical current into an ultrasonic backscatterthat encodes information related to the detected physiological signal;and emitting, from the first implantable device, the ultrasonicbackscatter that encodes the information related to the detectedphysiological signal; receiving, at an interrogator comprising one ormore ultrasonic transducers, the ultrasonic backscatter that encodes theinformation related to the detected physiological signal; emitting, fromthe interrogator, ultrasonic waves configured to power a secondimplantable device; receiving, at the second implantable device, theultrasonic waves configured to power the second implantable device;converting, at the second implantable device, energy from the ultrasonicwaves configured to power the second implantable device into anelectrical current that charges an energy storage circuit of the secondimplantable device; emitting, from the interrogator, ultrasonic wavesencoding a trigger signal; receiving, at the second implantable device,the ultrasonic waves encoding the trigger signal; and emitting, from thesecond implantable device, an electrical pulse that stimulates thetissue in response to the trigger signal.
 29. The method of claim 28,wherein the physiological signal comprises an electrophysiologicalsignal, a temperature, an analyte concentration, a pH, pressure, strain,or bioimpedance.
 30. The method of claim 28, wherein the tissuecomprises muscle tissue, an organ, or nervous tissue.